[bug report] El date picker format problem (format = "yyyy mm dd dd HH: mm") pop-up box displays the problem, and the day word appears in the tenth cell

### Element UI version
2.15.7

### OS/Browsers version
Windows10操作系统/谷歌浏览器

### Vue version
2.6.14

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

### Steps to reproduce
format和value-format 设置为 yyyy年MM月dd日HH:mm

### What is Expected?
弹框一个单元格（日期）为xxxx年xx月xx日 第二个为XX:XX

### What is actually happening?
弹框一个单元格（日期）为xxxx年xx月xx 第二个为日XX:XX