TST: Add regression test for Series index.map failing (#20990)
- [x] closes #20990
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

@WillAyd ping pls merge master Hello @Reksbril! Thanks for updating this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:



There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers: 

##### Comment last updated at 2019-11-18 15:55:56 UTC Thanks @Reksbril !