Return the name of all headers currently defined for the
        table.

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]