public function outputWithHeaders($filename = 'output.xls')
    {
        \header("Expires: " . date("r", 0));
        \header("Last-Modified: " . gmdate("r") . " GMT");
        \header("Content-Type: application/x-msexcel");
        \header("Content-Disposition: attachment; filename=" . $filename);
        echo $this->__toString();
    }