add test for fillna with column
- [X] tests added / passed

I noticed there was no test for pd.Series.fillna using a column, so I added one.
Hello @hasandiwan! Thanks for opening this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:

* In the file [`pandas/tests/series/methods/test_fillna.py`](https://github.com/pandas-dev/pandas/blob/f6059224456f6e287ec43524c6a1286fe398c548/pandas/tests/series/methods/test_fillna.py):

> [Line 24:38](https://github.com/pandas-dev/pandas/blob/f6059224456f6e287ec43524c6a1286fe398c548/pandas/tests/series/methods/test_fillna.py#L24): [E231](https://duckduckgo.com/?q=pep8%20E231) missing whitespace after ','
> [Line 24:41](https://github.com/pandas-dev/pandas/blob/f6059224456f6e287ec43524c6a1286fe398c548/pandas/tests/series/methods/test_fillna.py#L24): [E231](https://duckduckgo.com/?q=pep8%20E231) missing whitespace after ','
> [Line 32:1](https://github.com/pandas-dev/pandas/blob/f6059224456f6e287ec43524c6a1286fe398c548/pandas/tests/series/methods/test_fillna.py#L32): [W293](https://duckduckgo.com/?q=pep8%20W293) blank line contains whitespace

 are you sure this is not already solved by #5703 and PR #5705? The tests seem there already. closing this as stale