[Bug Report] el-table current-row-key can't highlight children

### Element UI version
2.13.0

### OS/Browsers version
macOS 10.15.3 / chrome 80.0.3987.149

### Vue version
2.6.10

### Reproduction Link
https://codepen.io/henrytsz/pen/OJVrQWM?editable=true

### Steps to reproduce
the first table current-row-key is parentId, can highlight, the second table current-row-key is childId, can't highlight

### What is Expected?
the childId can highlight

### What is actually happening?
the childId can't highlight