[Bug Report] select组件单选切换多选bug

### Element UI version
2.15.7

### OS/Browsers version
win10

### Vue version
2.6.14

### Reproduction Link
https://codepen.io/john418112/pen/bGooYmW

### Steps to reproduce
select组件控制multiple从false到true，单选中如果选择了选项，切换到true的时候，依然显示单选中的选中项，不影响结果，但是显示会出问题。出问题的参数为select组件中的selectedLabel，目前自己在项目中解决方式为
private get isMultiple():boolean{
let bool = this.tabIndex !== 7 && this.tabIndex !== 8
if(bool && this.$refs.speedRef){
(this.$refs.speedRef as any).selectedLabel = ""
}
return bool
}

### What is Expected?
希望解决

### What is actually happening?
目前自己解决的行为是 (this.$refs.speedRef as any).selectedLabel = ""；手动置空，希望以后更新这个bug