SpecialGWTooset clean-up & fix

* added 'allow-get' key to $_registered_modules and logic to determine
  the action to take when a module allows a get request or not
* added missing $this->outputHeader();
* removed extra $this->setHeaders();
* removed @return {void} comment

Change-Id: If571b9be56a2670616fe5d03642e6d5b621b7dbb