fix: add finalize callback in operator call

Closes ReactiveX#5357

spec/operators/finalize-spec.ts

@@ -193,4 +193,16 @@ describe('finalize operator', () => {
     subscription.unsubscribe();
     expect(order).to.deep.equal(['teardown', 'finalize']);
   });
+
+  it('should finalize after the teardown with synchronous completion', () => {
+    const order: string[] = [];
+    const source = new Observable<void>(subscriber => {
+      subscriber.complete();
+      return () => order.push('teardown');
+    });
+    source.pipe(
+      finalize(() => order.push('finalize'))
+    ).subscribe();
+    expect(order).to.deep.equal(['teardown', 'finalize']);
+  });
 });

src/internal/operators/finalize.ts

@@ -68,25 +68,8 @@ class FinallyOperator<T> implements Operator<T, T> {
   }
 
   call(subscriber: Subscriber<T>, source: any): TeardownLogic {
-    return subscribeWith(source, new FinallySubscriber(subscriber, this.callback));
-  }
-}
-
-/**
- * We need this JSDoc comment for affecting ESDoc.
- * @ignore
- * @extends {Ignored}
- */
-class FinallySubscriber<T> extends Subscriber<T> {
-  constructor(destination: Subscriber<T>, private callback?: () => void) {
-    super(destination);
-  }
-  unsubscribe() {
-    super.unsubscribe();
-    const { callback } = this;
-    if (callback) {
-      callback();
-      this.callback = undefined;
-    }
+    const subscription = subscribeWith(source, subscriber);
+    subscription.add(this.callback);
+    return subscription;
   }
 }