[Bug Report] el-tree set the disabled state, but the style of the content line is not disabled or selected

### Element UI version
2.15.1

### OS/Browsers version
Mac

### Vue version
2.6.12

### Reproduction Link
https://codepen.io/pen/?&prefill_data_id=5f00cf5d-1c27-43c5-9537-3c481dfaec8d

### Steps to reproduce
Set the disabled state, but the style of the content line is not disabled or selected

### What is Expected?
After the disabled state is set, the selected style is not added to the content line

### What is actually happening?
After setting the disabled status, the 'is current' style will be added to the content line