public function setNotModified() {

		$this->setStatusCode(self::HTTP_NOT_MODIFIED);
		$this->setContent(NULL);

		// remove headers that MUST NOT be included with 304 Not Modified responses

		foreach (['Allow', 'Content-Encoding', 'Content-Language', 'Content-Length', 'Content-MD5', 'Content-Type', 'Last-Modified'] as $header) {
			$this->headers->remove($header);
		}

		return $this;

	}