Use EmptyObserver instead of Subscribers.empty()

onError should be ignored after unsubscribe is called.

Author: zsxwing

src/main/java/rx/internal/operators/BufferUntilSubscriber.java

@@ -21,6 +21,7 @@
 import rx.Observer;
 import rx.Subscriber;
 import rx.functions.Action0;
+import rx.observers.EmptyObserver;
 import rx.observers.Subscribers;
 import rx.subjects.Subject;
 import rx.subscriptions.Subscriptions;
@@ -50,6 +51,9 @@
  */
 public class BufferUntilSubscriber<T> extends Subject<T, T> {
 
+    @SuppressWarnings("rawtypes")
+    private final static Observer EMPTY_OBSERVER = new EmptyObserver();
+
     /**
      * @warn create() undescribed
      * @return
@@ -92,7 +96,7 @@ public void call(final Subscriber<? super T> s) {
                 s.add(Subscriptions.create(new Action0() {
                     @Override
                     public void call() {
-                        state.observerRef = Subscribers.empty();
+                        state.observerRef = EMPTY_OBSERVER;
                     }
                 }));
                 boolean win = false;