Fix issue with JavaFxObservable$fromObservableSet() not emitting changes

Author: thomasnield

src/main/java/rx/javafx/sources/ObservableSetSource.java

@@ -13,6 +13,7 @@ public static <T> Observable<ObservableSet<T>> fromObservableSet(final Observabl
 
         return Observable.create((Observable.OnSubscribe<ObservableSet<T>>) subscriber -> {
             SetChangeListener<T> listener = c -> subscriber.onNext(source);
+            source.addListener(listener);
             subscriber.add(JavaFxSubscriptions.unsubscribeInEventDispatchThread(() -> source.removeListener(listener)));
         }).startWith(source).subscribeOn(JavaFxScheduler.getInstance());
     }