BUG: Period.to_timestamp not matching PeriodArray.to_timestamp
- [ ] closes #xxxx
- [x] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

With this change, I _think_ we can move to share some methods between Period and PeriodArray.