assign "spatial ref" variable attrs

[dcherian]

diffcrs.proj.assign_crs(spatial_ref="epsg:4326")

for a dataset without an existing spatial_ref, this will create a new spatial_ref variable with no attrs. This is confusing because if the spatial_ref variable was present, those existing attrs will be preserved (and possibly out-of-date).

I think it would be good to be consistent here --- either the spatial ref always has attributes (kept in sync with the index) or it never has attributes.