API Gateway with custom domain and base path returns 404

Upgrading from version 3.x to 4.x (Using Koa framework)

Api Gateway + custom domain + base path `job`

Event is coming as :
```
{
"resource": "/{proxy+}",
"path": "/job/status",
...
"pathParameters": {
"proxy": "status"
}
...
}
```
It seems `pathParameters.proxy` value is taken and `ctx.url` is set to `/job` instead of `/job/status` which ends up with 404.

@brett-vendia Is this somehow related to - https://github.com/vendia/serverless-express/blob/4b5ca7a5e90c7709b25b8fada30e13a76ed5c651/src/event-sources/utils.js#L8

Is there any way this stripBasePath to be configurable ?

从引用的行开始检查 src/event-sources/utils.js，跟踪在构建 Koa 请求 URL 时如何使用 pathParameters.proxy。使用自定义域名基础路径 `job` 重现所提供的 API Gateway 事件；当 `/job/status` 请求不再返回 404，并且可以按要求配置基础路径剥离行为时，即表示完成。

由索引模型根据 Issue 内容生成。