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

cc @jorisvandenbossche i dont know if this is the optimal way to do this, but it seems like there is a lot of share-ability here.