BUG: Preserve timezone in unaligned assignments
- [x] closes #12981 
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

A fix for #12981.  When doing a slice assign to a DataFrame where the
right-hand-side was timezone-aware datetime Series which required
realignment to perform the assignment, the timezone would be stripped
from the RHS on assignment.
