Breadcrumb.Item的重构出现了不兼容

[MatriXiao88]

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

Reproduction link

Steps to reproduce

1. 从页面上看到Mock Node后面少了一个分隔符/
2. 但如果切换到4.2.4版本的话是会有一个分隔符/

What is expected?

期望在demo场景下仍然有分隔符

What is actually happening?

demo场景下没有分隔符

Environment	Info
antd	4.5.2
React	16.11.0
System	win 10
Browser	chrome 83

---

这个现象是从4.3.0版本出现的，该版本对Breadcrumb使用hooks进行了重构，不兼容代码对比如下：

//  4.2.4 BreadcrumbItem.tsx定义了默认值
  static defaultProps = {
    separator: '/',
  };

// 4.3.0 BreadcrumbItem.tsx中没有重新在BreadcrumbItem.defaultProps 定义separator

[ant-design-bot]

Hello @Rhuaer. 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. Appreciate it advance and we are looking forward to your contribution!

你好 @Rhuaer, 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！

[lihqi]

ant-design/components/breadcrumb/BreadcrumbItem.tsx

Line 20 in 8c7f7b4

separator,

separator = '/',

这样也没什么问题吧