[Bug Report] cannot use el-input inside el-table

### Element UI version
2.15.7

### OS/Browsers version
Macos/Chrome and Safari Latest

### Vue version
2.6.14

### Reproduction Link
none

### Steps to reproduce
```

var Main = {
data() {
return {
tableData: ['aaa', 'bbb']
}
}
}
var Ctor = Vue.extend(Main)
new Ctor().$mount('#app')

```
### What is Expected?
Can edit el-input inside el-table!!!

### What is actually happening?
cannot edit string inside el-table