public function toString()
    {
        $string = '';
        $headers = $this->headers;
        if ($this->canSort()) {
            uksort($headers, [$this, 'sortHeaders']);
        }
        foreach ($headers as $collection) {
            foreach ($collection as $header) {
                if ($this->isDisplayed($header) || '' != $header->getFieldBody()) {
                    $string .= $header->toString();
                }
            }
        }

        return $string;
    }