REF: _is_compatible_with_other -> _can_union_without_object_cast
- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Changes the behavior in one test where ATM we behave differently for Series/ndarray than we do for Index arg