protected function _oauthRequest($url, $params) {
    if (!isset($params['access_token'])) {
      $params['access_token'] = $this->getAccessToken();
    }

    if (isset($params['access_token'])) {
      $params['appsecret_proof'] = $this->getAppSecretProof($params['access_token']);
    }

    // json_encode all params values that are not strings
    foreach ($params as $key => $value) {
      if (!is_string($value) && !($value instanceof CURLFile)) {
        $params[$key] = json_encode($value);
      }
    }

    return $this->makeRequest($url, $params);
  }