ENH: fill_value argument for shift #15486
Description
Feature Request
At the moment, the Series/DataFrame shift methods always fill with nans, which causes an upcast for boolean/integer types. In cases where there is an appropriate non-nan fill value, it would be useful to provide this value in shift, rather than having to correctly downcast the result. This would presumably also be more efficient than .shift().fillna() in those cases where no upcast is required.
Example Code
>>> pd.Series([True]).shift(fill_value=False)
0 False
dtype: bool