BUG: GroupBy with TimeGrouper sorts unstably
- [x] closes #7453 
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

Because it may affect to resample perf, alternative idea is to add `TimeGropuper` path to `GroupBy.first` and `last`.

hmm, ok, can you add a tests specifically for this sorting differential?
 Yep, added tests using `get_group`.
