[Bug Report] Enter causes switch to toggle before form submission

### Element UI version
2.4.6

### OS/Browsers version
All

### Vue version
2.5.17

### Reproduction Link
[https://jsfiddle.net/4r7qu50o/](https://jsfiddle.net/4r7qu50o/)

### Steps to reproduce
1. Open fiddle and focus on input.
2. Tab into the switch
3. Hit enter

### What is Expected?
The form should be submitted with the current value of the switch.

### What is actually happening?
The switch gets toggled and then the form submits, resulting in an unexpected value for the switch. The current behavior is inconsistent with browser/checkboxes.