[bug report] the selected event of the table under expand cannot be triggered after a column of the table is fixed

### Element UI version
2.15.1

### OS/Browsers version
chrome

### Vue version
2.6.12

### Reproduction Link
https://codepen.io/saojun1024/pen/MWpyOLK

### Steps to reproduce
1.先展开某一行
2.点击选中某一行

### What is Expected?
展开的表格勾选框会被全选

### What is actually happening?
展开的表格勾选框没有会被全选