[Bug Report] Switch component default value bug

### Element UI version
2.13.2

### OS/Browsers version
10.15.5 / CHROME 89.0

### Vue version
2.5.17

### Reproduction Link
https://codepen.io/MicroRrrr/pen/jOWZroZ

### Steps to reproduce
SWITCH COMPONENT

### What is Expected?
Using the switch component without setting alterative value, it should be read the value of False.

### What is actually happening?
When I used switch component in my project, I found that the default value of "False" can not be read if you don't click the switch like active it. However, if I set a alterative default value like 0 to replace false and 1 to replace true, it would be able to read the value without click the switch.