[bug report] the El tree component property 'default expanded keys' is not a default property, but a real-time value that will update the component view. It is recommended to change its name to' expanded keys'.

### Element UI version
2.13.2

### OS/Browsers version
windows/macOS

### Vue version
2.6.11

### Reproduction Link
https://codepen.io/chaoszsc/pen/RwrVmVo?editable=true%3Dhttps%3A%2F%2Felement.eleme.cn%2F

### Steps to reproduce
1.给可选择el-tree组件‘default-expanded-keys’设置响应参数
2.选中节点时更新‘default-expanded-keys’的参数
3.该选中节点被展开

### What is Expected?
‘default-expanded-keys’不应加default（默认）前缀，基于其实时更新节点属性，引发视图变更的功能，建议改为‘expanded-keys’

### What is actually happening?
默认属性值变更时，实时更新了视图