Standardize cast_str behavior in all datetimelike fill_value validators
- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

This will in turn allow us to simplify/de-duplicate a bunch of code since we now pass cast_str in all cases.
can you actually just remove this parameter entirely now? (e.g. its always true), or other cases? > can you actually just remove this parameter entirely now? (e.g. its always true), or other cases?

yes, planned for the next pr actually, since i need to rebase anyway, might as well now lgtm ping on green ping @jreback gentle ping; this is a blocker for long-awaited code de-duplication