set className in cascader, there will tow classname the same in dom

[wukong1995]

• I have searched the issues of this repository and believe that this is not a duplicate.

Reproduction link

https://codepen.io/wukong1995/pen/abZdVrb

Steps to reproduce

检查元素，查看dom节点

What is expected?

one class

What is actually happening?

tow class the same

Environment	Info
antd	4.7.0
React	16.13.1
System	macos
Browser	chrome lastest

[wukong1995]

[ant-design-bot]

Hello @wukong1995. We totally like your proposal/feedback, welcome to send us a Pull Request for it. Please send your Pull Request to proper branch (feature branch for the new feature, master for bugfix and other changes), fill the Pull Request Template here, provide changelog/TypeScript/documentation/test cases if needed and make sure CI passed, we will review it soon. We appreciate your effort in advance and looking forward to your contribution!

你好 @wukong1995, 我们完全同意你的提议/反馈，欢迎直接在此仓库 创建一个 Pull Request 来解决这个问题。请将 Pull Request 发到正确的分支（新特性发到 feature 分支，其他发到 master 分支），务必填写 Pull Request 内的预设模板，提供改动所需相应的 changelog、TypeScript 定义、测试用例、文档等，并确保 CI 通过，我们会尽快进行 Review，提前感谢和期待您的贡献！