Add test on complete signal for WebSocket

[segabriel]

WebSocket API: https://github.com/scalecube/scalecube-services/wiki/Web-Socket-API

Also it should work with local services which respond in another thread, example:

  @Override
  public Mono<String> one(String one) {
    return Mono.just(one);
  }

==>

  @Override
  public Mono<String> one(String one) {
    return Mono.delay(Duration.ofNanos(1), Schedulers.elastic()).thenReturn(one);
  }