Merge pull request #3625 from jreback/to_csv

BUG: Fix not consolidating before to_csv (GH3624_)
BUG: pivot fixing

BUG: Fix pivoting with ``nan`` in the index (GH3558_)

CLN: removed combine_from_indexers (not necessary) in lib.pyx

BUG: fixed unstack with an index with out any values for an entry