BUG: pd.to_datetime fixed (#28238)
- [ ] closes #28238
- [ ] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Fix for KeyError when trying to convert a series with unsorted float index to a DateTime 



@AntonioAndraues
@Vinigl

Here's some prints showing the fix. I just changed what @mroeschke suggested in https://github.com/pandas-dev/pandas/issues/28238#issuecomment-526772229

![bug_before](https://user-images.githubusercontent.com/13368721/64176002-e65c3500-ce32-11e9-9593-a978e5cb31b7.png)

![bug_after](https://user-images.githubusercontent.com/13368721/64176001-e65c3500-ce32-11e9-8463-272f81eb3476.png)
 I think @mroeschke was suggesting that change as a workaround in user's code, not as a change to pandas. Rather, we'll want to fix the bug that only surfaces with `cache=True`. closing as per comments