[Bug Report] Table statistics, fixed columns cannot drag horizontal scrollbars (table statistics, fixed columns cannot drag horizontal scrollbars)

### Element UI version
2.8.2

### OS/Browsers version
Windows/All

### Vue version
2.6.10

### Reproduction Link
https://codepen.io/anon/pen/LopRGB

### Steps to reproduce
Copy the following code to the HTML area（将以下代码拷贝至HTML区域）：
Fixed this column（将此列固定）
Duplicate several columns of data until horizontal scrolling occurs（多复制几列数据，直至能出现横向滚动）

### What is Expected?
Fixed columns can also drag scrollbars (horizontally)（固定列也可以拖动滚动条（横向））

### What is actually happening?
Fixed columns cannot drag scrollbars（固定列不能拖动滚动条）
Solution（解决方案）：
Modify the style in the current single page or globally, as follows（在当前的单页中修改样式或者在全局修改样式，具体代码如下）：
.el-table--scrollable-x>.el-table__body-wrapper{
z-index:3 !important;
background: transparent;
}
.el-table--scrollable-x>.el-table__body-wrapper>.el-table__body>tbody>tr{
background: transparent;
}