[Bug Report] When using a 2 digit date format, typing a 4 digit date truncates the year instead of using the last 2 digits

### Element UI version
2.14.0

### OS/Browsers version
Windows 10 / Chrome latest

### Vue version
2.6.12

### Reproduction Link
https://jsfiddle.net/6nj7ogk2/1/

### Steps to reproduce
Click in the date picker and type 1/1/1988 then hit enter.

### What is Expected?
It should be able to recognize the 4 digit year and use '88' (1988).

### What is actually happening?
Because the format is "MM/dd/yy", what happens is the '88' gets truncated and it uses '19' for the year which resolves to 2019