for::setUserAgent

public function setUserAgent($userAgent = null)
    {
        // Invalidate cache due to #375
        $this->cache = array();

        if (false === empty($userAgent)) {
            return $this->userAgent = $userAgent;
        } else {
            $this->userAgent = null;
            foreach ($this->getUaHttpHeaders() as $altHeader) {
                if (false === empty($this->httpHeaders[$altHeader])) { // @todo: should use getHttpHeader(), but it would be slow. (Serban)
                    $this->userAgent .= $this->httpHeaders[$altHeader] . " ";
                }
            }

            return $this->userAgent = (!empty($this->userAgent) ? trim($this->userAgent) : null);

        }
    }