public function setServerValue($key, $value)
    {
        if ($this->_immutable) {

            throw new Zend_Http_UserAgent_Exception(
                'The User-Agent device object has already been retrieved; the server array is now immutable'
            );
        }

        $server = $this->getServer(); // ensure it's been initialized
        $key    = strtolower($key);
        $this->_server[$key] = $value;
        return $this;
    }