We should consider to introduce a new AP method:

    `<U> HttpResponse<U> HttpResponse::map(Function<T,U> mapper);`

This way we could also promote the following:

    HttpResponse<W> resp =
         client.sendAsync(req, BodySubscribers.ofInputStream())
              .thenApplyAsync(r -> r.map(toJSON(W.class, r.body()))
              .join();

This would allow the caller to decide whether to performed the mapping in a synchronous or asynchronous dependent action, as well as allowing him to supply an executor if he so wishes.