[bug report] datepicker week date selection component. When the binding value is formatted as a timestamp, it will be garbled

### Element UI version
2.15.3

### OS/Browsers version
win10

### Vue version
2.6.14

### Reproduction Link
https://codepen.io/pangju666/pen/OJmgjZo

### Steps to reproduce
将datepicker组件的type属性设置为week，value-format属性设置为timestamp，此时无法正确回显选择的日期，通过打印v-model值可以得知此时的v-model值是乱码

### What is Expected?
返回选择的周起始日期的时间戳

### What is actually happening?
返回一串乱码