BUG: DatetimeIndex.freq can defer by ufunc

[sinhrks]

Follow up of #10638. DatetimeIndex.freq also can be changed when other arg is an ndarray.

As the same as #10638, freq is re-infered only when it exists. Otherwise, leave it as None even if the result can be on specific freq. This is based on other functions behavior as below:

idx = pd.DatetimeIndex(['2011-01-01', '2011-01-02', '2011-01-03'])
idx
# DatetimeIndex(['2011-01-01', '2011-01-02', '2011-01-03'], dtype='datetime64[ns]', freq=None)
idx.take([0, 2])
# DatetimeIndex(['2011-01-01', '2011-01-03'], dtype='datetime64[ns]', freq=None)

idx = pd.DatetimeIndex(['2011-01-01', '2011-01-02', '2011-01-03'], freq='D')
idx.take([0, 2])
# DatetimeIndex(['2011-01-01', '2011-01-03'], dtype='datetime64[ns]', freq='2D')

[jreback]

lgtm. merge on green.