mocking request loses original koa app instance, breaking resolving of req.subdomains, because req.app.subdomainOffset is undefined
`request.subdomains` getter depends on `request.app.subdomainOffset`, which can be configured after koa is instantiated:
- https://koajs.com/#settings

e.g. for hostname `app.example.com` original ctx.subdomains would resolve to ['app'] (with koa's default subdomainOffset = 2), and mocked req.subdomains would resolve to ['com', 'example', 'app']. 

(This is breaking passport authentication for us because we are resolving tenants from subdomains, and at the same time we are checking jwt's issuer if it matches the subdomain)

Of course I can workaround it, but nevertheless I'd consider it as a bug. What's interesting, koa-passport initially delegated original app object in `req.app` but then, probably with this change: https://github.com/rkusa/koa-passport/commit/0b7da657d31ec5910b558252b4351316bc67f28a it began to overwrite `app` with that almost-empty mock, which can only return `trust proxy` property and nothing besides that.