[Bug Report] el-table-column sortable="custom" does not trigger table header caret/arrows

### Element UI version
2.13.2

### OS/Browsers version
Safari 14.0 (15610.1.28.1.9, 15610), masOS Chrome 85.0.4183.121 (Official Build) (64-bit)

### Vue version
2.6.11

### Reproduction Link
https://jsfiddle.net/evilsideburns/yzn389ge/2/

### Steps to reproduce
Click the ascending and descending buttons.

### What is Expected?
That one of the following would allow the custom sortable to update the carets/arrows next to the Name column label:
- dynamically setting default-sort
- calling this.$refs.table.sort(), with or without this.$refs.table.doLayout()

Also expected: that documentation (https://element.eleme.io/#/en-US/component/table) would clarify how to use sortable="custom".

### What is actually happening?
The carets/arrows next to the Name column label never change.