[Bug Report] el-select showing value instead of label after update options and binding value in watch method

### Element UI version
2.13.0

### OS/Browsers version
macOS/Chrome80.0.3987.163

### Vue version
2.6.11

### Reproduction Link
https://codepen.io/v32vdqtgegf/pen/ZEbzawB

### Steps to reproduce
1. pick an option
2. after 1sec delay it will show the updated value which label are expected

### What is Expected?
show updated label

### What is actually happening?
show value