public function setUserAgent($userAgent)
    {
        if (is_null($userAgent)) {
            foreach ($this->getUaHttpHeaders() as $altHeader) {
                if (isset($this->httpHeaders[$altHeader])) {
                    $userAgent .= $this->httpHeaders[$altHeader].' ';
                }
            }
        }

        return $this->userAgent = $userAgent;
    }