TypeError: next is not a function

When I run the authentication sample code from the [documentation](https://gabrielcsapo.github.io/node-git-server/code/index.html) and then try to clone an existing repository, I get the following error. I've changed nothing about the sample code other than the directory and the port number, so I'd assume this is an issue with the source code.

> server.js:13
> next();
> ^
>
> TypeError: next is not a function
> at Git.authenticate (server.js:13:9)
> at Git.res.statusCode (node_modules/node-git-server/lib/git.js:307:38)
> at next (node_modules/node-git-server/lib/git.js:425:32)
> at Git.handle (node_modules/node-git-server/lib/git.js:427:9)
> at Server. (node_modules/node-git-server/lib/git.js:450:16)
> at Server.emit (node:events:390:28)
> at parserOnIncoming (node:_http_server:951:12)
> at HTTPParser.parserOnHeadersComplete (node:_http_common:128:17)
>
> Node.js v17.2.0

从链接文档中的身份验证示例开始，将其 callback 的使用方式与 node_modules/node-git-server/lib/git.js 中报告的第 307、425-450 行进行比较。在 Node.js 17.2.0 上重现 clone 操作，并追踪 next 不是函数的原因；当现有 repository 能够在不出现此 TypeError 的情况下完成 clone 时，即表示完成。

由索引模型根据 Issue 内容生成。