private function _addHeaderRequirements()
    {
        $this->_headerRequirements = array();
        foreach ($this->getRequirements() as $key => $value) {
            if (0 === strpos($key, 'HTTP-')) {
                $this->_headerRequirements[substr($key, 5)] = $value;
            }
        }

        return $this;
    }