[Bug Report] Form validation + transform: sanitized value not propagated back to model

### Element UI version
2.13.2

### OS/Browsers version
Chrome 83.0.4103.106 / Mac OS 10.14.5

### Vue version
2.6.11

### Reproduction Link
https://jsfiddle.net/nomatter/6x4ybpsu/17/

### Steps to reproduce
1. Open the provided fiddle
2. enter email
3. click "send invite" button on the form
4. check console logs

### What is Expected?
The sanitized value is applied to the underlying model

### What is actually happening?
During form validation, the sanitized/transformed value is not assigned back to the underlying model.