[Bug Report] Date Picker week model issue,when I init date as 2021-01-03, it format result is right,but the selected range in dropdown is 2020-12-27 to 2021-01-02

### Element UI version
2.0.11

### OS/Browsers version
chrome

### Vue version
2.5.16

### Reproduction Link
https://jsfiddle.net/L1pj9avo/2/

### Steps to reproduce
type set as week, init value set as new new Date(2021,0,3) you will see 2021-01-03,but when click picker, it shows you that your selected range is 2020-12-27 to 2020-01-02

### What is Expected?
picker should show selected range is 2021-01-03 to 2021-01-09

### What is actually happening?
selected range is 2020-12-27 to 2020-01-02