Because of the previous awkward factoring of Origin we had two
implementations of Origin serializing and creation. Move the
implementation of DOMURL::url_origin into URL::origin, and
instead use the implemenation of URL::Origin::serialize for
serialization (replacing URL::serialize_origin).
This happens to fix 8 URL subtests as the two implemenations had
diverged, and URL::serialize_origin was previously missing the spec
changes of: whatwg/url@eee49fd and whatwg/url@fff33c3
