[Bug Report] el-checkbox problem with internal checkbox state

### Element UI version
2.8.2

### OS/Browsers version
Windows 10, Chrome 74

### Vue version
2.6.10

### Reproduction Link
https://codepen.io/anon/pen/EzgLVx

### Steps to reproduce
1) Click multiple times on the el-checkbox, the state will not change (normal, because the setter of the v-model is empty).

2) Look at the console log: the setter receives multiple times the value "true". This is correct.

3) Change the Vue.js version to vue@2.6.0-beta.1 or newer (change the script in the HTML part of the codepen), and try again: the setter receives "true" and "false" alternating, like the internal state of the el-checkbox is actually changing, even if it should not.

### What is Expected?
The el-checkbox should generate a "setter" event that has always the value "true" (trying to go checked), even with recent versions of Vue.js (2.6.0 ).

### What is actually happening?
The setter for the el-checkbox model receives alternating "true" and "false" values, even if the checkbox state is actually always "unchecked".

It looks like a this problem is definitely triggered by a change in Vue.js (1868561: fix checkbox event edge case in Firefox), but I'm not sure if it's a Vue regression or simply a bug in el-checkbox that is now apparent because of the Vue change.

Better knowledge of the internal working of el-checkbox is needed to tell the cause of the problem.