[Bug Report] the bug when i use component with el-tree's attribute icon-class element

### Element UI version
2.4.5

### OS/Browsers version
window 10 professional

### Vue version
2.5.2

### Reproduction Link
https://jsfiddle.net/mmx38qxw/

### Steps to reproduce
tep 1 create a el-tree tag

tep 2 click parent node element the tree ,it will open because parent node element has children ,and now this node element's icon is exists
tep 3 click node element which id is 2 ,the node element will not open because it without children ,and now this node element's icon should hide but it is't;

### What is Expected?
i add the icon-class attribute for component el-tree,when i click node element which without children ,it's icon should be hideen。
actually it is exists

### What is actually happening?
when i click node element which without children ,it's icon still exists.