ENH: should shift return same dtype objects as input?

[jreback]

see the example in #2759

should certain operations, e.g. shift preserve the dtype of the output object,
simulated like this (rather than return an upcasted version)
this obviously is easier if we had an integer NA

would have to add a fillna keyword to shift to make this work
(so we don't have to guess on the fill value for integer/bool types)

mask.astype('O').shift().fillna(0).astype(mask.dtype)

[jreback]

did what we could via #2708, #2871, #2838