"__ob__" can be enumerated

emmmm...my English is so poor....
```js
//with out setup()
{
  data(){
    return { x：{} };
  },
  mounted(){
    for (let p in this.x) { console.log(p) }//print nothing
  },
}
```

```js
{
  setup(){
    let x = {};
    onMounted(() => {
      for (let p in x) { console.log(p) }//print "__ob__"
    });
    return {x};
  }
}
```