/**
  * @param RequestInterface $request
  * @return string[]
  */
 protected function buildHeaders(RequestInterface $request)
 {
     // If the auth token has been provided as an attribute of the
     // request then we use it
     $authToken = $this->authToken;
     if (isset($request->getAttributes()['auth_token'])) {
         $authToken = $request->getAttributes()['auth_token'];
     }
     return ['Authorization' => $authToken];
 }