BUG: Fix ``.diff`` on datelike and timedelta operations (GH3100_)
Merge pull request #3554 from jreback/GH3100

BUG: Fix .diff() on datelike and timedelta operations (GH3100_)
BUG: ``combine_first`` not returning the same dtype in cases where it can (GH3552_)

ENH: ``convert_objects`` now accepts a ``copy`` parameter (defaults to ``True``)

PTF
Merge pull request #3555 from jreback/GH3552

BUG: ``combine_first`` not returning the same dtype in cases where it can (GH3552)
BUG: fix typeerror caused by spurious call to len on frame multiindex