add one more piece of test logic

Author: thomasnield

src/test/java/rx/javafx/sources/JavaFxObservableTest.java

@@ -371,6 +371,10 @@ public void testcompositeObservableFinite() {
 
             assertTrue(emissions.size() == 4);
 
+            compositeObservable.add(source2);
+
+            assertTrue(emissions.size() == 6);
+
             latch.countDown();
         });