docs: def next_header_class(self):
          '''Return class of next header, if known.'''
          if self._next_header_class_key == '':
              return None
          key = getattr(self, self._next_header_class_key)
          rv = self._next_header_map.get(key, None)
          if rv is None:
              log_warn("No class exists to handle next header value {}".format(key))
          return rv