ENH: Implement mode(dropna=False)
- [x] closes #17534
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

There are other mode tests in ``tests/test_algos.py`` but I'm not sure if I should test the ``dropna`` parameter in this file too. The tests seem very similar to those in ``tests/series/test_analytics.py``.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20779?src=pr&el=h1) Report
> Merging [#20779](https://codecov.io/gh/pandas-dev/pandas/pull/20779?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/7647969df247727afdcbcfa52169f4436d6ef377?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/20779/graphs/tree.svg?height=150&width=650&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/20779?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #20779      +/-   ##
==========================================
+ Coverage   91.84%   91.85%   +<.01%     
==========================================
  Files         153      153              
  Lines       49540    49546       +6     
==========================================
+ Hits        45501    45509       +8     
+ Misses       4039     4037       -2
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.25% <100%> (ø)` | :arrow_up: |
| #single | `41.87% <22.22%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/20779?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/series.py](https://codecov.io/gh/pandas-dev/pandas/pull/20779/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc2VyaWVzLnB5) | `94.12% <100%> (ø)` | :arrow_up: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/20779/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.22% <100%> (ø)` | :arrow_up: |
| [pandas/core/algorithms.py](https://codecov.io/gh/pandas-dev/pandas/pull/20779/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYWxnb3JpdGhtcy5weQ==) | `94.81% <100%> (+0.31%)` | :arrow_up: |
| [pandas/core/arrays/categorical.py](https://codecov.io/gh/pandas-dev/pandas/pull/20779/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL2NhdGVnb3JpY2FsLnB5) | `95.69% <100%> (+0.01%)` | :arrow_up: |
| [pandas/core/sparse/array.py](https://codecov.io/gh/pandas-dev/pandas/pull/20779/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc3BhcnNlL2FycmF5LnB5) | `91.38% <0%> (+0.06%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20779?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/20779?src=pr&el=footer). Last update [7647969...6f14357](https://codecov.io/gh/pandas-dev/pandas/pull/20779?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Hello @reidy-p! Thanks for updating the PR.













Cheers ! There are no PEP8 issues in this Pull Request. :beers: 

##### Comment last updated on May 31, 2018 at 10:39 Hours UTC @reidy-p looks really good.  thanks @reidy-p !