ENH: Implement mode(dropna=False)#20779

Merged

jreback merged 8 commits intopandas-dev:masterfrom

reidy-p:mode_dropna

May 31, 2018

Commits on May 30, 2018

Put series mode tests in class and remove uncessary sort_values and reset_index
reidy-p
committed

Commits on May 31, 2018

Merge branch 'master' into PR_TOOL_MERGE_PR_20779
jreback
committed