docs: @RequestMapping(value="/{subscription}", method=GET, params="hub.mode=subscribe")
  	public @ResponseBody String verifySubscription(
  			@PathVariable("subscription") String subscription,
  			@RequestParam("hub.challenge") String challenge,
  			@RequestParam("hub.verify_token") String verifyToken) {
  		logger.debug("Received subscription verification request for '" + subscription + "'.");
  		return tokens.containsKey(subscription) && tokens.get(subscription).equals(verifyToken) ? challenge : "";
  	}