TST: Add testcase for the fix to bug #4476.
TST: Add testcase for the fix to bug #4476.

Add a testcase `test_fillvalue_datetime_timedelta` to class
`TestFillingValues` for the fix to bug #4476.  See commit
216fd17 and pull request #5455.

andsor/numpy,andsor/numpy,andsor/numpy,andsor/numpy