docs: public Observable<ServiceResponse<Void>> registerWithServiceResponseAsync(String userName) {
          if (userName == null) {
              throw new IllegalArgumentException("Parameter userName is required and cannot be null.");
          }
          if (this.client.apiVersion() == null) {
              throw new IllegalArgumentException("Parameter this.client.apiVersion() is required and cannot be null.");
          }
          final String registrationCode = null;
          RegisterPayload registerPayload = new RegisterPayload();
          registerPayload.withRegistrationCode(null);
          return service.register(userName, this.client.apiVersion(), this.client.acceptLanguage(), registerPayload, this.client.userAgent())
              .flatMap(new Func1<Response<ResponseBody>, Observable<ServiceResponse<Void>>>() {
                  @Override
                  public Observable<ServiceResponse<Void>> call(Response<ResponseBody> response) {
                      try {
                          ServiceResponse<Void> clientResponse = registerDelegate(response);
                          return Observable.just(clientResponse);
                      } catch (Throwable t) {
                          return Observable.error(t);
                      }
                  }
              });
      }