[bug report] tab tree, select all checebox, row children can't be selected / deselected.

### Element UI version
2.12.0

### OS/Browsers version
win10/chrome 78.0.3904.70

### Vue version
2.6.10

### Reproduction Link
https://codepen.io/pen/?editors=1010

### Steps to reproduce
At table tree,
1. when user clicks the checkbox in table header， the checkbox in row children hasn't be selected;
2.first ,user selects row children, then click the checkbox in table header to select all. At last,deselect the checkbox in table header, the checkbox in row children's status hasn't be changed,it‘s still at selected status.
（在树形数据的表格中，点击切换表头全选checkbox状态，有树形数据的表格行checkbox状态不能联动。
先选择树形数据子表格的checkbox，再点击全选checkbox，当取消表格所有行全选状态时，树形数据子表格的checkbox状态任然是选择状态。）

### What is Expected?
when click the checkbox in tabel header, the all row of table should be selected/deselected.
（1.点击切换表头全选checkbox状态的时候，表格所有行应该处于选中或者未选中状态。
2.点击切换有children 的行的checkbox状态时，默认切换该行所有children相应的checkbox的状态。）

### What is actually happening?
when click the checkbox in tabel header, the all row of table isn't selected/deselected.
（点击切换表头全选checkbox状态的时候，children子表格checkbox状态没有与全选状态联动。）