BUG: DataFrame.to_string with formatters, header and index False
closes #13032 
- [x] tests added / passed - added test specific to format bug
- [x] passes `pep8radius master --diff`
- [x] whatsnew entry - not needed

Found this bug experimenting with formatters. First pull request to pandas, but I believe guidelines are quite clear. I can explain what was happening in more detail if that is necessary.
