docs: def headers(self):
          """ Return the name of all headers currently defined for the
          table. """
          if self._headers is None:
              query = CellQuery()
              query.max_row = '1'
              feed = self._service.GetCellsFeed(self._ss.id, self.id,
                                                query=query)
              self._headers = feed.entry
          return [normalize_header(h.cell.text) for h in self._headers]