Make to_csv return a string in case no buffer is supplied.

Fixes issue #6061.
FIX use selected_obj rather the obj throughout groupby

TST dont ignore subselection in groupby
PERF #6570 patch by @jreback
Merge pull request #6570 from hayd/groupby_selected_obj

FIX use selected_obj rather the obj throughout groupby
Merge pull request #6406 from filmor/patch-2

Make to_csv return a string if no path or buffer is provided.
BUG: Bug in fillna with method = bfill/ffill and datetime64[ns] dtype (GH6587)
Merge pull request #6588 from jreback/date_fill

BUG: Bug in fillna with method = bfill/ffill and datetime64[ns] dtype (GH6587)
ENH: including offset/freq in Timestamp repr (#4553)
Merge pull request #6575 from rosnfeld/issue_4553

ENH: including offset/freq in Timestamp repr (#4553)
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
Remove the special case for empty dataframes in to_html.

Fixes issue #6062.