protected function initDefaultHeaders()
    {
        // add this header to prevent .php request to be cached
        $this->addHeader(HttpProtocol::HEADER_EXPIRES, '19 Nov 1981 08:52:00 GMT');
        $this->addHeader(HttpProtocol::HEADER_CACHE_CONTROL, 'no-store, no-cache, must-revalidate, post-check=0, pre-check=0');
        $this->addHeader(HttpProtocol::HEADER_PRAGMA, 'no-cache');

        // set per default text/html mimetype
        $this->addHeader(HttpProtocol::HEADER_CONTENT_TYPE, 'text/html');
    }