fix(NextTickAction): fix unsubscription behavior

- fix clearImmediate behavior prevents nextTickScheduler throws if it's
being used multiple time

closes ReactiveX#582

src/schedulers/NextTickAction.ts

@@ -4,8 +4,7 @@ import ImmediateAction from './ImmediateAction';
 import Action from './Action';
 
 export default class NextTickAction<T> extends ImmediateAction<T> {
-
-  id: number;
+  private id: any;
 
   schedule(state?: any): Action {
     if (this.isUnsubscribed) {
@@ -21,7 +20,7 @@ export default class NextTickAction<T> extends ImmediateAction<T> {
     if (!scheduler.scheduled) {
       scheduler.scheduled = true;
       this.id = Immediate.setImmediate(() => {
-        this.id = void 0;
+        this.id = null;
         this.scheduler.scheduled = false;
         this.scheduler.flush();
       });
@@ -30,8 +29,7 @@ export default class NextTickAction<T> extends ImmediateAction<T> {
     return this;
   }
 
-  unsubscribe() {
-
+  unsubscribe(): void {
     const id = this.id;
     const scheduler = this.scheduler;
 
@@ -40,10 +38,11 @@ export default class NextTickAction<T> extends ImmediateAction<T> {
     if (scheduler.actions.length === 0) {
       scheduler.active = false;
       scheduler.scheduled = false;
-      if (id) {
-        this.id = void 0;
-        Immediate.clearImmediate(id);
-      }
+    }
+
+    if (id) {
+      this.id = null;
+      Immediate.clearImmediate(id);
     }
   }
 }