[Bug Report] The border of the table using span-method is misaligned

### Element UI version
0.1.0

### OS/Browsers version
windows10/Google Chrome 78.0.3904.108

### Vue version
2.5.2

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

### Steps to reproduce
open "https://element.eleme.cn/#/zh-CN/component/table" and move to “合并行或列”

### What is Expected?
The border of the header and the border of the body are on the same vertical line

### What is actually happening?
The border of the header and the border of the body are not on the same vertical line