protected function _setCacheControl()
    {
        $control = '';
        foreach ($this->_cacheDirectives as $key => $val) {
            $control .= $val === true ? $key : sprintf('%s=%s', $key, $val);
            $control .= ', ';
        }
        $control = rtrim($control, ', ');
        $this->_setHeader('Cache-Control', $control);
    }