[bug report] when the El table container is scaled with the transform: scale() attribute, gaps appear between the header cells, causing the background color to penetrate

### 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
容器使用transform: scale()属性，表头单元格之间出现缝隙

### What is Expected?
表头单元格、内容单元格之间没有缝隙

### What is actually happening?
表头单元格、内容单元格之间存在缝隙，导致背景色透穿以及el-table的border模式（带单元格分隔线）效果不明显