Support `casting` kwarg in COO.astype

[dcherian]

Is your feature request related to a problem? Please describe.
COO.astype doesn't have a casting kwarg unlike numpy and dask. There seem to be other unsupported (by sparse) kwargs on the numpy side: https://numpy.org/doc/stable/reference/generated/numpy.ndarray.astype.html

Describe the solution you'd like
Implement the casting kwarg.

Describe alternatives you've considered
None

Additional context
This is breaking some xarray tests: pydata/xarray#4314

[hameerabbasi]

Hi! Thanks for the report. I've fixed this in #392.

[dcherian]

really quick! Thanks @hameerabbasi