code: def _update_response_body(self, resource):
          
          rpr = self._get_response_representer(resource)
          # Set content type and body of the response.
          self.request.response.content_type = \
                                  rpr.content_type.mime_type_string
          rpr_body = rpr.to_bytes(resource)
          self.request.response.body = rpr_body
docs: Creates a representer and updates the response body with the byte
          representation created for the given resource.