to_csv `headers` kwarg now works regardless of `index` kwarg
Addresses issue https://github.com/pydata/pandas/issues/6186

pls add a release note referring to the issue
 Sorry I'm new to the PANDAS style guide: There's a note in the unit test---should I also put a note in `format.py`?
 no
need a note in doc/source/release.rst
 Just added it---look good?
 this looks good, and DRYer to boot. Awesome!
