docs: def _resolve_serializer(self, serializer):
          """
          Resolve the given serializer.
  
          :param serializer: The serializer to resolve
          :type serializer: str or Serializer
  
          :rtype: Serializer
          """
          if isinstance(serializer, Serializer):
              return serializer
  
          if serializer in self._serializers:
              return self._serializers[serializer]
  
          raise RuntimeError('Unsupported serializer')