[bug report] the length of El table header is too long, which affects the rendering of the list

### Element UI version
2.15.5

### OS/Browsers version
windows 10

### Vue version
2.6.10

### Reproduction Link
https://github.com

### Steps to reproduce
el-table 头部标题字符长度为6，列表渲染的高度第一次渲染的时候计算的高度不正确，待页面重新刷新时才正常

### What is Expected?
列表渲染的高度 不受头部标题字符长度的影响

### What is actually happening?
修改