fix bfill, ffill and pad when calling with df.interpolate with column…
… dtype string (#33956)

- [x] closes #33956 
- [x] closes #12918 
- [x] closes #29146 
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry