[Bug Report] The clearSort method of the el-table does not trigger sort-change event

### Element UI version
2.8.2

### OS/Browsers version
win10 / chrome 72

### Vue version
2.6.10

### Reproduction Link
https://codepen.io/anon/pen/yWNBPr?editors=1010

### Steps to reproduce
1、click table header trigger sort-change event
2、click clearSort button does not trigger sort-change event

### What is Expected?
The clearSort method of the el-table can trigger sort-change event

### What is actually happening?
The clearSort method of the el-table does not trigger sort-change event