BUG: DateOffset weekday around DST produces unexpected results. fixes #5175

closes #5175

Wound up working on this in preparation for the DST change next week. Here's a first pass at a solution and a test.
this seems like a bug to me, eg. ``t + pd.offsets.Hour(9)`` == ``t + pd.DateOffset(hours=9) == t + timedelta(hours=9)`` (and the first is prob how most users do this). Are their tests for the former anywhere? can you point them out.