Make Series.shift always a copy?
Right now, `Series.shift(0)` will just return the series. Shifting for all other periods induces a copy:

```python
In [1]: import pandas as pd

In [2]: a = pd.Series([1, 2])

In [3]: a.shift(1) is a
Out[3]: False

In [4]: a.shift(0) is a
Out[4]: True
```

Should we defensively copy on `0` as well, for a consistent user experience?

https://github.com/pandas-dev/pandas/blob/e669fae0762d901e61f7af84fc3b5181848d257d/pandas/core/generic.py#L8084-L8086