BUG: FooIndex.insert casting datetimelike NAs incorrectly
- [ ] closes #xxxx
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry


any perf issues? can you rebase > any perf issues?

we'll take a hit in cases where we now cast to object, but id call those bugs in master.

> can you rebase

done + green