Add queryString: Optional<String> method to Request and NativeRequest

[jknack]

The way to get the queryString today is a bit awkward:

get("/514", req -> {
      Map<String, Mutant> params = req.params().toMap();
      return req.path() + "?" + params.entrySet().stream()
          .map(e -> e.getKey() + "=" + e.getValue().value()).collect(Collectors.joining("&"));
    });

On next release this will provided via Request.queryString method

get("/514", req -> {
      return req.path() + req.queryString().map(q -> "?" + q).orElse("");
    });

See #514

[nikos]

Perfect, thanks a lot for taking care!