Fix in date conversion param handling
Previously this was trying to update any column name that had changed in `self._convert_dates`. If you had a column name that was non-date that was being changed, it would throw an error.

 - [ ] closes #xxxx
 - [ ] tests added / passed
 - [ ] passes ``git diff upstream/master --name-only -- '*.py' | flake8 --diff``
 - [ ] whatsnew entry
