[Bug Report] default-expanded-keys doesn't work with lazy

### Element UI version
2.13.0

### OS/Browsers version
firefox 60

### Vue version
2.6.11

### Reproduction Link
https://jsfiddle.net/mkr0x6cw/1/

### Steps to reproduce
Use the tree with Lazy .
I have part of the tree in the data prop and the remainder of the data that forms the tree that should be lazy loaded when specific nodes are selected.
I use the default-expanded-key with the id of a node which is in the pre-loaded part of the tree (aka in data prop).

### What is Expected?
When the tree is displayed, I want the tree to be expanded to the pre-selected node.

### What is actually happening?
With the Lazy option, the tree doesn't expand.
If I remove the Lazy option from the tree, then the tree expands as expected.