cascader级联选择异步请求option设置value显示空白 。

- [x] I have searched the [issues](https://github.com/vueComponent/ant-design-vue/issues)         of this repository and believe that this is not a duplicate.
        
        
### What problem does this feature solve?
设置:value="['xxx']"页面input显示空白 。

### What does the proposed API look like?
vue: 2.6   os: 10.14  chrome: 73