The [bug report] checkbox group change event could not get the value correctly

### Element UI version
2.13.0

### OS/Browsers version
chrome Version 78.0.3904.108

### Vue version
2.6.10

### Reproduction Link
https://codepen.io/chess126/pen/rNaeNYb?&editable=true

### Steps to reproduce
点击一个checkbox

### What is Expected?
:value写法也能正常获取更新后的值

### What is actually happening?
v-model写法, change事件可以获取更新后的值;
:value写法, 无法获取更新后的值