docs: def hide_defaults(self):
          """Removes fields' values that are the same as default values."""
          # use list(): self.fields is modified in the loop
          for k, v in list(six.iteritems(self.fields)):
              v = self.fields[k]
              if k in self.default_fields:
                  if self.default_fields[k] == v:
                      del self.fields[k]
          self.payload.hide_defaults()