Accept Promise as return type for transformRequest

[ahocevar]

This pull request enables transformRequest() to return a Promise resolving to a Request or url string, instead of just a Request or an url string. This allows users that need to add auth headers to obtain those headers asynchronously:

transformRequest: async (url, type) => {
  return new Request(
    url,
    {
      headers: await getAuthHeaders()
    }
  );
}