[Bug Report] reassign expand-row-keys would keep the previous expanded rows still expanded

### Element UI version
2.13.0

### OS/Browsers version
macOS 10.14.3 / chrome 版本 80.0.3987.132

### Vue version
3.0.0-alpha.7

### Reproduction Link
https://codepen.io/xiaobeizi/pen/RwPjbqK?editable=true

### Steps to reproduce
At first set the expand-row-keys to ['3']，after 3 seconds reassign the expand-row-keys to ['1']

### What is Expected?
only row['1'] is expanded

### What is actually happening?
both row['3'] and row['1'] are expanded