/**
  * Get the token type that grants will return in the HTTP response.
  *
  * @return ResponseTypeInterface
  */
 protected function getResponseType()
 {
     if ($this->responseType instanceof ResponseTypeInterface === false) {
         $this->responseType = new BearerTokenResponse();
     }
     $this->responseType->setPrivateKey($this->privateKey);
     return $this->responseType;
 }