fix(buffer): subscribe to source and closingNotifier in proper order

In buffer operator subscribe to source observable first, so that when
closingNotifier emits value, all source values emited before land in buffer

Closes #1610

spec/operators/buffer-spec.ts

@@ -219,4 +219,17 @@ describe('Observable.prototype.buffer', () => {
     expectObservable(a.buffer(b).take(1)).toBe(expected, expectedValues);
     expectSubscriptions(b.subscriptions).toBe(bsubs);
   });
+
+  it('should work with filtered source as closingNotifier', () => {
+    const values = {0: 0, 1: 1, 2: 2, 3: 3, 4: 4, 5: 5, 6: 6, 7: 7, 8: 8};
+
+    const source =    hot('-0-1-2-3-4-5-6-7-8-|', values);
+    const expected =      '-a---b---c---d---e-|';
+
+    const expectedValues = {a: [0], b: [1, 2], c: [3, 4], d: [5, 6], e: [7, 8]};
+    const filteredSource = source.filter(x => x % 2 === 0);
+
+    const result = source.buffer(filteredSource);
+    expectObservable(result).toBe(expected, expectedValues);
+  });
 });

src/operator/buffer.ts

@@ -1,5 +1,6 @@
 import { Operator } from '../Operator';
 import { Subscriber } from '../Subscriber';
+import { TeardownLogic } from '../Subscription';
 import { Observable } from '../Observable';
 
 import { OuterSubscriber } from '../OuterSubscriber';
@@ -47,8 +48,11 @@ class BufferOperator<T> implements Operator<T, T[]> {
   constructor(private closingNotifier: Observable<any>) {
   }
 
-  call(subscriber: Subscriber<T[]>, source: any): any {
-    return source.subscribe(new BufferSubscriber(subscriber, this.closingNotifier));
+  call(subscriber: Subscriber<T[]>, source: any): TeardownLogic {
+    const bufferSubscriber = new BufferSubscriber(subscriber);
+    const subscription = source.subscribe(bufferSubscriber);
+    bufferSubscriber.subscribeToClosingNotifier(this.closingNotifier);
+    return subscription;
   }
 }
 
@@ -60,8 +64,11 @@ class BufferOperator<T> implements Operator<T, T[]> {
 class BufferSubscriber<T> extends OuterSubscriber<T, any> {
   private buffer: T[] = [];
 
-  constructor(destination: Subscriber<T[]>, closingNotifier: Observable<any>) {
+  constructor(destination: Subscriber<T[]>) {
     super(destination);
+  }
+
+  subscribeToClosingNotifier(closingNotifier: Observable<any>) {
     this.add(subscribeToResult(this, closingNotifier));
   }