When the [bug report] container uses the transform: scale() attribute to zoom, there will be a gap between the cell and the header, which will cause the background color to pass through.

### Element UI version
2.13.0

### OS/Browsers version
windows 10/Chrome 89.0.4389.114

### Vue version
2.6.11

### Reproduction Link
https://codepen.io/ziyoung/pen/rRKYpd123

### Steps to reproduce
1、el-table不使用border属性
2、el-table的容器使用ttransform:scale属性进行缩放设置，表头单元格以及内容单元格之间会出现缝隙，容器背景色会透出来

### What is Expected?
单元格之间没有缝隙

### What is actually happening?
单元格之间有缝隙