Calling shift on a DatetimeIndex of length 0 returns an Index instead of a DatetimeIndex

[ssanderson]

Minimal repro:

In [5]: pandas.__version__
Out[5]: '0.16.0-151-gcef3c85'

In [7]: DatetimeIndex([]).shift(3, 'H')
Out[7]: Index([], dtype='object')

[ssanderson]

(This happens because DatetimeIndex delegates to Index.shift, which then calls the Index.new with an empty list, and it can't infer that the output should be a DatetimeIndex.)

[ssanderson]

This is fixed by #9904.

[sinhrks]

This has been already fixed in current master (maybe by #11211?). Adding tests then close.