Return class of next header, if known.

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