BUG: (GH3593) fixed a bug in the incorrect conversion of datetime64[ns] in combine_first
Merge pull request #3595 from jreback/combine_first_timestamp

BUG: (GH3593) fixed a bug in the incorrect conversion of datetime64[ns]  in combine_first
Merge pull request #3585 from cpcloud/fillna-raise-on-value-is-list-3435

raise on fillna passed a list or tuple
ENH: add regex functionality to DataFrame.replace
add default of None to to_replace
add ability to pass regex as to_replace regex
Remove cruft
more tests and add ability to pass regex and value