[Bug Report] 动态table-column，数据改变不会更新表头的内容

### Element UI version
2.14.1

### OS/Browsers version
macos/chrome 86.0.4240.198

### Vue version
2.6.11

### Reproduction Link
https://codepen.io/mountainp/pen/eYzXaMo

### Steps to reproduce

{{ c.label }}

data() {
return {
columns: [{label: '1'}, {label: '2'}]
};
}

mounted() {
setTimeout(() => {
this.columns = [{label: '11'}, {label: '22'}];
}, 5000);
}

### What is Expected?
表头的内容应该更新为11，22

### What is actually happening?
表头内容未进行更新