fix(throttle): now properly trailing throttles for individual values

fixes #2859

spec/operators/throttle-spec.ts

@@ -7,6 +7,8 @@ declare const hot: typeof marbleTestingSignature.hot;
 declare const cold: typeof marbleTestingSignature.cold;
 declare const expectObservable: typeof marbleTestingSignature.expectObservable;
 declare const expectSubscriptions: typeof marbleTestingSignature.expectSubscriptions;
+declare const time: typeof marbleTestingSignature.time;
+declare const rxTestScheduler: typeof marbleTestingSignature.rxTestScheduler;
 
 const Observable = Rx.Observable;
 
@@ -372,5 +374,14 @@ describe('Observable.prototype.throttle', () =>  {
       expectSubscriptions(e1.subscriptions).toBe(e1subs);
       expectSubscriptions(e2.subscriptions).toBe(e2subs);
     });
+
+    it('should work for individual values', () => {
+      const s1 = cold('---------x---------x--------x------|');
+      const n =  cold('----(n|)');
+      const exp =     '-------------x---------x--------x--|';
+
+      expectObservable(s1.throttle(() => n, { trailing: true }))
+        .toBe(exp);
+    });
   });
 });

src/operators/throttle.ts

@@ -95,22 +95,17 @@ class ThrottleSubscriber<T, R> extends OuterSubscriber<T, R> {
   }
 
   protected _next(value: T): void {
-    if (this.throttled) {
-      if (this._trailing) {
-        this._hasTrailingValue = true;
-        this._trailingValue = value;
-      }
-    } else {
+    if (this._trailing) {
+      this._hasTrailingValue = true;
+      this._trailingValue = value;
+    }
+    if (!this.throttled) {
       const duration = this.tryDurationSelector(value);
       if (duration) {
         this.add(this.throttled = subscribeToResult(this, duration));
       }
       if (this._leading) {
         this.destination.next(value);
-        if (this._trailing) {
-          this._hasTrailingValue = true;
-          this._trailingValue = value;
-        }
       }
     }
   }