[Bug Report] Tree component：checked items will be unchecked when dragged

### Element UI version
2.12.0

### OS/Browsers version
Mac / Chrome 77

### Vue version
2.6.10

### Reproduction Link
https://codepen.io/komomo/pen/NWWbVXv

### Steps to reproduce
1, checked 3-2-1

2, drag 3-2-1

3, observed that 3-2-1 was unchecked

### What is Expected?
Drag and drop behavior does not affect the check status

### What is actually happening?
Drag and drop behavior led to unchecking

Start with the linked CodePen reproduction and the Tree component's drag-and-drop and checked-state handling. Reproduce the sequence of checking 3-2-1 and then dragging it, then trace the relevant component entry points in the repository. Done means dragging checked items leaves their check status unchanged and the behavior is covered by a regression test.

Written by the indexing model from the issue text.