API: Series.to_csv(path=None) should return string to match DataFrame.to_csv()
Addresses #8215. Currently, `Series.to_csv(path=None)` returns `None`. The `DataFrame` method returns a string, and already had this behaviour mentioned in the docstring, so it should be OK to make the `Series` method match this.

minor comment. pls squash. ping when green
 @jreback Added those comments and squashed, should be good to go.
 @onesandzeroes thanks!
