Added tests for generic observer pattern

observer/src/main/java/com/iluwatar/observer/generic/Observable.java

@@ -22,6 +22,10 @@ public abstract class Observable<S extends Observable<S, O, A>, O extends Observ
     this.observers.add(observer);
   }
 
+  public void removeObserver(O observer) {
+    this.observers.remove(observer);
+  }
+
   /**
    * Notify observers
    */