FIX filter selects selected columns

TST for selected groupby add resample ohlc and filter
Merge pull request #6593 from hayd/groupby_filter_filter

FIX filter selects selected columns
EHN/FIX: Add na_position parameter to DataFrame.sort. Fixes GH3917
TST: Skip mergesort test if `np.argsort` raises TypeError on any mergesort.