ENH: Add unique and nunique to GroupBy whitelist

Series.unique and Series.nunique are useful methods for querying the
unique elements (or number of unique elements) in each group when used
in conjunction with DataFrame.GroupBy.  This commit addresses
pydata/pandas#6146.
ENH: Updated tests for series GroupBy whitelist

Added tests for fix to pydata/pandas#6146.
Merge pull request #6147 from bburan-galenea/bburan/groupby-series-unique

ENH: Add unique and nunique to GroupBy whitelist