BUG: Patch handling of keep_default_na=False
Patches very buggy behavior of `keep_default_na=False` whenever `na_values` is a dict

* Respect `keep_default_na` for column that doesn't exist in `na_values` dictionary
* Don't crash / break when `na_value` is a scalar in the `na_values` dictionary.

In addition, clarifies documentation on the handling of the keep `keep_default_na` parameter with respect to `na_filter` and `na_values`.

Closes #19227.

cc @neilser
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19260?src=pr&el=h1) Report
> Merging [#19260](https://codecov.io/gh/pandas-dev/pandas/pull/19260?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/04a6815f7da0f6d879725d553c7a45f47bb4a15c?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/19260/graphs/tree.svg?height=150&width=650&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/19260?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #19260      +/-   ##
==========================================
+ Coverage   91.56%   91.56%   +<.01%     
==========================================
  Files         148      148              
  Lines       48874    48878       +4     
==========================================
+ Hits        44751    44755       +4     
  Misses       4123     4123
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.94% <100%> (ø)` | :arrow_up: |
| #single | `41.69% <21.42%> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/19260?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/parsers.py](https://codecov.io/gh/pandas-dev/pandas/pull/19260/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL3BhcnNlcnMucHk=) | `95.49% <100%> (+0.01%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19260?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/19260?src=pr&el=footer). Last update [04a6815...63faf8d](https://codecov.io/gh/pandas-dev/pandas/pull/19260?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @jreback : The OSX builds on Travis seemed to have stalled completely (all of the other tests have been done for a LONG time now). thanks @gfyoung 