[bug report] element UI El tree setcheckedkeys individual data cannot be completely cleared

### Element UI version
2.13.2

### OS/Browsers version
macOs 10.14.5/ 83.0.4103.97

### Vue version
2.5.2

### Reproduction Link
https://codepen.io/mafeixiong/pen/vYLGjxR

### Steps to reproduce
加载tree后，清空数据使用setCheckedKeys([])个别数据无法清空，数据结构value没有重复的
this.$nextTick(() => {
this.$refs.tree.setCheckedKeys([]);
console.log("clear");
});

### What is Expected?
期望数据被清空

### What is actually happening?
个别数据无法被清空