Fix #28552 Add test for DataFrame min/max preserve timezone
- [x] closes #28552
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry _(not applicable - no functionality added)_

Here's a test discussed in #28552. `mean` and other aggregates except for `min` and `max` are not covered since they don't support datetimes - it's a nan being returned.

Hello @dair-targ! 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 2020-05-12 11:57:56 UTC @mroeschke thanks for review! I hope I've addressed all the issues - would you mind to review it again? thanks @dair-targ 