Removes fields' values that are the same as default values.

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()