[Bug Report] date-picker,type='daterange',value is timestamp,value-format="yyyy-MM-dd" 这种组合搭配会报错

### Element UI version
2.15.8

### OS/Browsers version
window/chrome

### Vue version
2.6.5

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

### Steps to reproduce
1、open Reproduction Link
2、分别切换 v-model="timestampFormat" 和 v-model="dateFomrat"
timestampFormat 会报错

### What is Expected?
fix bug

### What is actually happening?
TypeError: dateStr.match is not a function