Adding 'n/a' to list of strings denoting missing values
 - [x] closes #16078
 - [x] tests added / passed
 - [x] passes ``git diff upstream/master --name-only -- '*.py' | flake8 --diff``
 - [x] whatsnew entry

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16079?src=pr&el=h1) Report
> Merging [#16079](https://codecov.io/gh/pandas-dev/pandas/pull/16079?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/db419bfad9afb363156f23acd0810f01e1b2c9a8?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/16079/graphs/tree.svg?src=pr&token=eZ4WkYLtcO&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/16079?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #16079   +/-   ##
=======================================
  Coverage   90.75%   90.75%           
=======================================
  Files         161      161           
  Lines       51095    51095           
=======================================
  Hits        46370    46370           
  Misses       4725     4725
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.59% <ø> (ø)` | :arrow_up: |
| #single | `40.16% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/16079?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/common.py](https://codecov.io/gh/pandas-dev/pandas/pull/16079?src=pr&el=tree#diff-cGFuZGFzL2lvL2NvbW1vbi5weQ==) | `69.91% <ø> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16079?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16079?src=pr&el=footer). Last update [db419bf...eef689a](https://codecov.io/gh/pandas-dev/pandas/pull/16079?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Wow. That codecov report makes little sense. I suppose this is ok.

@jorisvandenbossche  Done. @jorisvandenbossche @TomAugspurger ?

lgtm. I think +1... My only hesitation is that we've run into problems before with people un-wantingly getting stuff converted to NaN, but since we already treat N/A as a nan value, I think this is fine. @jorisvandenbossche thoughts? Yeah, if we would start again, certainly +1 on adding it to the default list to parse as missing. But if someone has csv files with actual valid 'n/a' values, we will for sure break his/her code ... Difficult to assess however if that would occur. 
I will say, if you are +1, I won't object :-) Small comment on the PR: I would list this in the backwards incompatible API changes section, not enhancements Since this is technically an API change, I think we should wait till 0.20.1. Sorry for not getting this in before the RC @chrisfilo  > Since this is technically an API change, I think we should wait till 0.20.1.

Small correction, I think this should be 0.21.0, as 0.20.1 shouldn't contain API changes. So how should we proceed? Is there a separate branch for 0.21.0 I should send thi PR to?  > So how should we proceed? Is there a separate branch for 0.21.0 I should send thi PR to?

It will still be merged into master, just not until after we've finished the 0.20.0 release (this week sometime). Once that's out, I'll add a `whatsnew/0.21.0` and ask you to move your release note over there. Also, need more tests!  Try passing in data containing your new `na_value`.  an you rebase Done. @gfyoung the test you requested is here: https://github.com/pandas-dev/pandas/pull/16079/files#diff-6e435422c67fa1384140f92110fb69a7R73 Awesome!  @jreback @TomAugspurger might it be prudent to start sharing the na_values list somehow so that we don't have to do maintenance in two different files?  Not blocking this PR but jut a thought. > Awesome! @jreback @TomAugspurger might it be prudent to start sharing the na_values list somehow so that we don't have to do maintenance in two different files? Not blocking this PR but jut a thought.

yeah that is certainly possible, if you'd like to do an issue/PR I'm getting the following test error, but I don't see how it could be related to the changes I made. Please advise:

```
=================================== FAILURES ===================================
_ TestCommonIOCapabilities.test_write_fspath_all[to_hdf-writer_kwargs3-tables] _

self = <pandas.tests.io.test_common.TestCommonIOCapabilities object at 0x7ff8c30fdc88>
writer_name = 'to_hdf', writer_kwargs = {'key': 'bar', 'mode': 'w'}
module = 'tables'

    @pytest.mark.parametrize('writer_name, writer_kwargs, module', [
        ('to_csv', {}, 'os'),
        ('to_excel', {'engine': 'xlwt'}, 'xlwt'),
        ('to_feather', {}, 'feather'),
        ('to_hdf', {'key': 'bar', 'mode': 'w'}, 'tables'),
        ('to_html', {}, 'os'),
        ('to_json', {}, 'os'),
        ('to_latex', {}, 'os'),
        ('to_msgpack', {}, 'os'),
        ('to_pickle', {}, 'os'),
        ('to_stata', {}, 'os'),
    ])
    def test_write_fspath_all(self, writer_name, writer_kwargs, module):
        p1 = tm.ensure_clean('string')
        p2 = tm.ensure_clean('fspath')
        df = pd.DataFrame({"A": [1, 2]})
    
        with p1 as string, p2 as fspath:
            pytest.importorskip(module)
            mypath = CustomFSPath(fspath)
            writer = getattr(df, writer_name)
    
            writer(string, **writer_kwargs)
            with open(string, 'rb') as f:
                expected = f.read()
    
            writer(mypath, **writer_kwargs)
            with open(fspath, 'rb') as f:
                result = f.read()
    
>           assert result == expected
E           AssertionError: assert b'\x89HDF\r\n...0\x00\x00\x00' == b'\x89HDF\r\n\...0\x00\x00\x00'
E             At index 2620 diff: 164 != 163
E             Use -v to get the full diff
``` Do you get this failure consistently?  And with which parameters is it failing?  (I presume `to_csv` but want to double check) Actually it's not `to_csv`, but `to_hdf`. If you could trigger rebuild on Circle we can check if this is failing consistently (sorry I don't have permissions to do it): https://circleci.com/gh/pandas-dev/pandas/2031#tests/containers/2 @chrisfilo : Neither do I (@jreback ?) 😢 However, I have been noticing lately that `CircleCI` has been acting up on us and failing in random places like this.  I'm beginning to wonder if `CircleCI` is useful for our testing purposes compared to Travis and Appveyor. ping can you rebase this Here you go.

PS Hint for keeping contributors happy - merge conflicting PRs in chronological order so people will not be punished with resolving conflicts for no apparent reason.  > PS Hint for keeping contributors happy - merge conflicting PRs in chronological order so people will not be punished with resolving conflicts for no apparent reason.

hah!

yeah we do leave space in the whatsnew so that we won't have conflicts to begin with.

we have lots of PR's. It is an iterative process. I am a bit biased as I look for recently updated ones that are green. Then I (or others) may make comments, causing another round of changes. The more responsive people are, the quicker things get merged. And of course the more they rebase the less conflicts occur.

:.

lgtm. ping on green. > PS Hint for keeping contributors happy - merge conflicting PRs in chronological order so people will not be punished with resolving conflicts for no apparent reason.

this actually is not fair. more fair is merging responsive PR's. They are more likely to be ready. We have long running PR's which need rebasing, which pro-active authors do. 

Sure when we have a queue and/or are trying to get out a release things get a bit jumbled... I believe in this case the more considerate thing to do would've been to read the [comments](https://github.com/pandas-dev/pandas/pull/16079#issuecomment-302888648), hit the rebuild button on circle, wait for green and merge this first. Recency of the last comment is a poor proxy of responsiveness if the contributor is blocked on faulty CI. This incredibly simple PR was ready to merge 12 days ago.

Ultimately it's is the faulty CI (or over reliance on it) that is causing the frustrations. I understand that there is a shit tonne of PRs in this project (truly remarkable), but I'm trying to give you a unique perspective of a project newcomer and an ability to improve your practices. Overall goal is to make the community better and more efficient. > I believe in this case the more considerate thing to do would've been to read the comments, hit the rebuild button on circle, wait for green and merge this first. Recency of the last comment is a poor proxy of responsiveness if the contributor is blocked on faulty CI. This incredibly simple PR was ready to merge 12 days ago.

@chrisfilo well you are assuming we are contantly searching for PR's that are ready to merge :> this is not the case. rather we DO get a ping and then react, or I do scan things on ready to go PR's. I DO prioritize all green PR's as those are most likely to be ready. Sure a CI failure happens, and certainly if pinged we would restart if possible, but if it didn't get restarted then its a red X, and it appears like the PR is non-responsive. Of could the writer can always do a ``git commit -C HEAD --all --amend`` and push to force a rebuild :>

 > pandas/tests/io/parser/na_values.py:73:80: E501 line too long (83 > 79 characters)

flake issue You were [pinged](https://github.com/pandas-dev/pandas/pull/16079#issuecomment-302892117).
[Twice](https://github.com/pandas-dev/pandas/pull/16079#issuecomment-305571589).

But you are right that I could've just done `git commit -C HEAD --all --amend`.

