protected function normalizeAccessToken($access_token) {
    if (!isset($access_token['token_type'])) {
      $access_token['token_type'] = 'Bearer';
    }
    if (!isset($access_token['expires_at']) && isset($access_token['expires_in'])) {
      $access_token['expires_at'] = $access_token['expires_in'] + $access_token['request_time'];
    }
    return $access_token;
  }