BUG: na_rep given precedence in to_html
- [x] closes #13828
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

~~I know new tests are appreciated from the get-go, but it's the end of the day for me. I'll add new tests and the appropriate whatsnew entry tomorrow.~~
