BUG: Handle infinite values correctly in Series.argmax
- [ X] closes #13595
- [X ] tests added / passed
- [X ] passes `git diff upstream/master | flake8 --diff`
- [X ] whatsnew entry

@matthewwillian You have some PEP8 issues (see the travis log at the bottom: https://travis-ci.org/pydata/pandas/jobs/144182270), and a failing test.
 Okay, thank you. I will take a look.
 @matthewwillian Can you update/rebase?
 It's great if you can include the fix for `argmin` for `-inf`.

```
pd.Series([1, 3, -np.inf]).argmin()
0
```
 can you rebase
 can you rebase / update?
 can you rebase and move release notes to 0.19.2
 can you rebase / update closing, but if you want to update, pls comment.