[Bug Report] i use v-for in el-table-column，when columnsArray changed, the table can't render right

### Element UI version
2.15.6

### OS/Browsers version
mac

### Vue version
2.6.14

### Reproduction Link
https://codepen.io/a602120/pen/mdMmKNo?editors=1010

### Steps to reproduce
when i changed my columns in settimeout, table not render

### What is Expected?
table render wright when columns change

### What is actually happening?
table not work when when i changed my columns in settimeout