[Bug Report] <El Table> When the column data is obtained Asynchronously, the default sorting column and highlight arrow cannot be set correctly

### Element UI version
2.15.0

### OS/Browsers version
MacOS 10.15.3 / Chrome 88.0.4324.96

### Vue version
2.6.12

### Reproduction Link
https://codepen.io/XDB/pen/qBqOGam

### Steps to reproduce
See codepen and can be compared:

Async data: https://codepen.io/XDB/pen/qBqOGam

Static data: https://codepen.io/XDB/pen/eYBpaEZ

### What is Expected?
Whether data are asynchronous or synchronous date, columns should be in descending order, arrows should be highlighted.

### What is actually happening?
The default sorting column cannot be set correctly and the arrow is not highlighted.