BUG: Fix downcast argument for DataFrame.fillna()
 - [X] closes #15277 
 - [X] tests added / passed
 - [x] passes git diff upstream/master | flake8 --diff
 - [X] whatsnew entry

needs tests, pls add for both series & dataframe (in pandas/tests/series/test_missing.py and similar for frame).

add alongside the existing tests

pls add a whatsnew note (bug fixes for 0.20.0) # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15278?src=pr&el=h1) Report
> Merging [#15278](https://codecov.io/gh/pandas-dev/pandas/pull/15278?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/9ddba8dc811184c49d9bc4df5dc97d505345ec23?src=pr&el=desc) will **increase** coverage by `-0.01%`.


```diff
@@            Coverage Diff             @@
##           master   #15278      +/-   ##
==========================================
- Coverage   86.33%   86.32%   -0.01%     
==========================================
  Files         139      139              
  Lines       51149    51157       +8     
==========================================
+ Hits        44157    44162       +5     
- Misses       6992     6995       +3
```


| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/15278?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/generic.py](https://codecov.io/gh/pandas-dev/pandas/compare/9ddba8dc811184c49d9bc4df5dc97d505345ec23...1b594a950fb180c5a5b3fee3b2b2228b7d187cd0?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ2VuZXJpYy5weQ==) | `96.33% <100%> (ø)` | :white_check_mark: |
| [pandas/core/common.py](https://codecov.io/gh/pandas-dev/pandas/compare/9ddba8dc811184c49d9bc4df5dc97d505345ec23...1b594a950fb180c5a5b3fee3b2b2228b7d187cd0?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29tbW9uLnB5) | `91.02% <ø> (-0.34%)` | :x: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/compare/9ddba8dc811184c49d9bc4df5dc97d505345ec23...1b594a950fb180c5a5b3fee3b2b2228b7d187cd0?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `82.54% <ø> (-0.19%)` | :x: |
| [pandas/indexes/category.py](https://codecov.io/gh/pandas-dev/pandas/compare/9ddba8dc811184c49d9bc4df5dc97d505345ec23...1b594a950fb180c5a5b3fee3b2b2228b7d187cd0?src=pr&el=tree#diff-cGFuZGFzL2luZGV4ZXMvY2F0ZWdvcnkucHk=) | `98.57% <ø> (ø)` | :white_check_mark: |
| [pandas/tseries/base.py](https://codecov.io/gh/pandas-dev/pandas/compare/9ddba8dc811184c49d9bc4df5dc97d505345ec23...1b594a950fb180c5a5b3fee3b2b2228b7d187cd0?src=pr&el=tree#diff-cGFuZGFzL3RzZXJpZXMvYmFzZS5weQ==) | `96.59% <ø> (ø)` | :white_check_mark: |
| [pandas/indexes/base.py](https://codecov.io/gh/pandas-dev/pandas/compare/9ddba8dc811184c49d9bc4df5dc97d505345ec23...1b594a950fb180c5a5b3fee3b2b2228b7d187cd0?src=pr&el=tree#diff-cGFuZGFzL2luZGV4ZXMvYmFzZS5weQ==) | `96.17% <ø> (+0.01%)` | :white_check_mark: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15278?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/15278?src=pr&el=footer). Last update [9ddba8d...1b594a9](https://codecov.io/gh/pandas-dev/pandas/compare/9ddba8dc811184c49d9bc4df5dc97d505345ec23...1b594a950fb180c5a5b3fee3b2b2228b7d187cd0?el=footer&src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments). lgtm. though you are going to fail linting because of indentation :< ping on green Is it OK now or some additional changes are needed? Should I squash all the commits into a single one? thanks @albertvillanova 

squashing is really no longer necessary as our merging process will squash (and retain the commit links).