[Bug Report] When next post form after first post. checkbox button component default value is losed

### Element UI version
2.13.1

### OS/Browsers version
Chrome 86.0.4240.183 ,Ubuntu 20.04

### Vue version
2.6.12

### Reproduction Link
https://github.com/tu6ge/laravel-form-item

### Steps to reproduce
git clone git@github.com:tu6ge/laravel-form-item.git

cd laravel-form-item

git checkout element

composer install

composer test-browser

you can find CheckboxButtonTest run failed .

### What is Expected?
I want to post correct data when second submit form.

### What is actually happening?
But ,I post data is wrong in second submit form.