Make to_csv return a string if no path or buffer is provided.
Fixes #6061.

this needs a test to validate that the return value is in fact a string.
 I added a test and reverted to the `path_or_buf` argument.
 @jorisvandenbossche @cpcloud think we need any more tests on this?
 can you add a release note (API section on this)
 Done.
 thanks!
