[Bug Report] file.response in upload hooks is not reactive

### Element UI version
2.13.2

### OS/Browsers version
all

### Vue version
2.6.12

### Reproduction Link
https://codepen.io/caikan/pen/ExyNoaL

### Steps to reproduce
1. upload 2 files

### What is Expected?
console outupt:
```
responses [undefined]
responses ["{err:0}"]
responses ["{err:0}", undefined]
responses ["{err:0}", "{err:0}"]
```

### What is actually happening?
console outupt:
```
responses [undefined]
responses ["{err:0}", undefined]
```