Cannot assign to a NaN variable

### Version
2.7.0

### Reproduction link
[codepen.io](https://codepen.io/andychyl/pen/KKowLWj)







### Steps to reproduce
data() {
  return {
    test: NaN
  }
}

created(){
  this.test = 10
  console.log(this.test) // output:NaN
}

### What is expected?
10

### What is actually happening?
NaN