BUG: TimedeltaIndex raising ValueError when boolean indexing (#14946)
 - [x] closes #14946
 - [x] tests added / passed
 - [x] passes ``git diff upstream/master | flake8 --diff``
 - [x] whatsnew entry

Implementing patch suggested by @jreback [here](https://github.com/pandas-dev/pandas/issues/14946#issuecomment-268666453) that prevents raising a `ValueError` when boolean indexing with a `TimedeltaIndex`