FIX: timeseries asfreq would drop the name of the index, closes #9854
it seems that it is open discussion whether reindex should preserve the name of the index.
(Towards the end of https://github.com/pydata/pandas/issues/6552)

My proposal for now is to set the name of the new index created by the asfreq function.

merged via https://github.com/pydata/pandas/commit/39fa180cf6b861981871b4a61bf0f3a31eaa2b98

thanks!
