code: def get_header(self):
          
          raw_header = yield get_arg(self, 1)
          if not self.serializer:
              raise tornado.gen.Return(raw_header)
          else:
              header = self.serializer.deserialize_header(raw_header)
              raise tornado.gen.Return(header)
docs: Get the header value from the response.
  
          :return: a future contains the deserialized value of header