BUG: Error while saving DataFrame with TimedeltaIndex to .csv #10833
Fix in accordance with https://github.com/pydata/pandas/issues/10833

needs some tests. use the example you gave. add into `test_frame.py`. needs a round-trip tests. Note that you will have to read it as `object` dtype and the run it thru `to_timedelta` to compare against the original.

pls also add a whatsnew note.
 It would be nice to refactor `test_frame.py` code... Btw, there are two existing assert methods: `self.assertXXX` and `assert_frame_equal`. I've used the first one as it seems like the second is to be reworked.
 @soupault yes of course `test_frame` could use a refactor. but that's another PR!
 thank you sir!
