code: public HttpService orElse(Service<HttpRequest, HttpResponse> nextService) {
          requireNonNull(nextService, "nextService");
          return new OrElseHttpService(this, nextService);
      }
docs: Creates a new {@link HttpService} that tries this {@link HttpFileService} first and then the specified
  {@link HttpService} when this {@link HttpFileService} does not have a requested resource.
  
  @param nextService the {@link HttpService} to try secondly