[Bug Report] element tree : the leaf nodes will be all checked when the lazy is true and this default-checked-keys is continuously

### Element UI version
2.13.0

### OS/Browsers version
Mozilla/5.0 (Macintosh; Intel Mac OS X 10_14_1) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/75.0.3770.100 Safari/537.36

### Vue version
2.6.10

### Reproduction Link
https://codepen.io/51code/pen/mdyJPKW?&editable=true

### Steps to reproduce
set the default-checked-keys like [2,3] or [2,3,4]

### What is Expected?
nodes [2,3,4] will be checked;

### What is actually happening?
nodes [2,3,4,5] will be checked;