@NotNull
    public <T> ResponseHandler<T> toSingleObject(@NotNull Class<T> cls) {
      return new GsonResponseHandler<>(this,
                                       s -> myGson.fromJson(s, cls),
                                       r -> myGson.fromJson(r, cls),
                                       () -> null);
    }
@NotNull
    public <T> ResponseHandler<T> toSingleObject(@NotNull Class<T> cls)