BUG: explicity change nan -> NaT when assigning to datelike dtypes
closes #4292 

essentially was doing a assignment of `np.nan` to a numpy array of dtype `datetime64[ns]`
which works correct (set's the value to `iNaT`) on little endian, for some reason on big
endian doesn't work, explicity change the `nan` to `iNaT`
