missing parameter in DataArray.str.get

[keewis]

While working on #4286 I noticed that the docstring of DataArray.str.get claims to allow passing a default value in addition to the index, but the python code doesn't have that parameter at all.
I think the default value is a good idea and that we should make the code match the docstring.

[mathause]

Similarly str.wrap does not pass on its kwargs

xarray/xarray/core/accessor_str.py

Line 654 in 7daad4f

def wrap(self, width, **kwargs):