Return the alias name for the relative header or None if nonexist.

def has_alias(self, header):
        """Return the alias name for the relative header or None if nonexist."""
        try:
            # Force to lower case (issue #1126)
            return self._limits[self.plugin_name + '_' + header.lower() + '_' + 'alias'][0]
        except (KeyError, IndexError):
            # logger.debug("No alias found for {}".format(header))
            return None