docs: def schema_is_registered_for_subject(self, subject, schema):
          """
          Returns True if the given schema is already registered under the
          given subject.
          """
          data = json.dumps({'schema': json.dumps(schema)})
          res = requests.post(self._url('/subjects/{}', subject), data=data, headers=HEADERS)
          if res.status_code == 404:
              return False
          raise_if_failed(res)
          return True