fix(bufferWhen): onComplete of closings determine buffers

Change bufferWhen operator to allow its closing Observables to close the
buffer whenever the closing Observable emits 'complete'. This is in
agreement with functionality in RxJS 4 buffer operator. Important for
the test 'Observable.prototype.bufferWhen should emit buffers using
varying empty delayed closings'.

src/operators/bufferWhen.ts

@@ -92,6 +92,6 @@ class BufferClosingNotifierSubscriber<T> extends Subscriber<T> {
   }
 
   _complete() {
-    // noop
+    this.parent.openBuffer();
   }
 }