docs: def _merge_meta(self, encoded_meta, meta):
          """
          Merge new meta dict into encoded meta. Returns new encoded meta.
          """
          new_meta = None
  
          if meta:
              _meta = self._decode_meta(encoded_meta)
              for key, value in six.iteritems(meta):
                  if value is None:
                      _meta.pop(key, None)
                  else:
                      _meta[key] = value
              new_meta = self._encode_meta(_meta)
  
          return new_meta