BUG: Using DatetimeIndex.date with timezone returns incorrect date #2…

- [ ] closes #21230, closes https://github.com/pandas-dev/pandas/issues/21267
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Added 2 tests for DatetimeIndex.date in tests/indexes/datetimes/test_datetime.py. Please let me know if I should move them somewhere else.