add test for fillna with column

[hasandiwan]

• tests added / passed

I noticed there was no test for pd.Series.fillna using a column, so I added one.

[pep8speaks]

Hello @hasandiwan! Thanks for opening this PR. We checked the lines you've touched for PEP 8 issues, and found:

• In the file pandas/tests/series/methods/test_fillna.py:

Line 24:38: E231 missing whitespace after ','
Line 24:41: E231 missing whitespace after ','
Line 32:1: W293 blank line contains whitespace

[attack68]

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

[jreback]

closing this as stale