REF: share .astype code for astype_nansafe + TDA.astype
- [ ] closes #xxxx
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

This changes TDI.astype and TDA.astype in the cast with no-NAs to return float64 instead of int64, matching Series behavior.