Fix to_csv of DataFrame with single level multiindex.
### What's this PR do?
Fixes the output of to_csv to show the value instead of the tuple when a ``DataFrame`` or ``Series`` is constructed using a ``MultiIndex`` with one level.  Although, I'm not sure if it's more desirable to require a MultiIndex to be instantiated with more than one level in the future. 

- [X] closes #19589
- [X] tests added / passed
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [X] whatsnew entry
