after/complete handler must work when going async

[jknack]

after((req, rsp, result) -> {
  return Results.ok("<" + result.get() + ">");
});

get("/", promise(deferred -> {
   executor.execute(() -> {
     deferred.resolve("foo");
   });
}));

GET / produces <foo>

Until now, after/complete callback didn't work in async mode