BUG: GH10209 fix bug where date_format in to_csv is sometimes ignored
closes #10209

When datetimes are not dates only i.e. have fractional day values, date_format passed to to_csv is ignored.  

This might be a silly question: Would I need to rebase after the PR passed the build test but has become stale? 
 you need to move the release note to v0.16.2.txt
 Ah. I see. Will do. Thanks!
 merged via https://github.com/pydata/pandas/commit/282746ae3c05edf7eac5c9583a17aa53ea12b7da

thanks!
