[bug report] about the lazy: load = "load" attribute of El table, close it after opening it, and then open it again without loading the data in it

### Element UI version
2.15.6

### OS/Browsers version
windows10

### Vue version
2.6.14

### Reproduction Link
https://jsfiddle.net/mmx38qxw/cout

### Steps to reproduce
点击下拉小箭头 之后加载 然后关闭 再加载

### What is Expected?
完成这个bug的修复

### What is actually happening?
loadOrToggle: function loadOrToggle(row) {
this.assertRowKey();
var _states5 = this.states,
lazy = _states5.lazy,
treeData = _states5.treeData,
rowKey = _states5.rowKey;

var id = getRowIdentity(row, rowKey);
var data = treeData[id];
if (lazy && data && 'loaded' in data && !data.loaded) {
this.loadData(row, id, data);
} else {
this.toggleTreeExpansion(row);
data.loaded = false
}
},

在else里面加上
data.loaded = false