Collection full_copy and clone don't actually deepcopy

[evetion]

When I clone or full_copy a Collection, its extra_fields, are not deepcopied. Mutating the copy will mutate the original:

from pystac import Collection, Extent, SpatialExtent, TemporalExtent

data = {"test": "1"}
a = Collection(id="test", description="test", extra_fields=data, extent=Extent(spatial=SpatialExtent(bboxes=[0,0,0,0]), temporal=TemporalExtent(intervals=[0,0])))

b = a.full_copy()
b.extra_fields.pop("test")
a.extra_fields  # {}

This probably also happens to Catalogs and to more fields than just extra_fields as well.