Improvment: OkOrNotFound controller base class method.

[rcollette]

I think it would be a better example for your readers to have a controller base class method like:

private static final NOT_FOUND = ResponseEntity.notFound().build();

protected Mono<ResponseEntity<T>> OkOrNotFound(Mono<T> result){
  return result.map(ResponseEntity::ok).defaultIfEmpty(NOT_FOUND);
}

In use it's now just

return OkOrNotFound(result);