notFound option does not respect router prefix

[IlyaSemenov]

notFound option does not respect router prefix.

It is called for any non-matched path, regardless of the prefix, meaning it can't be used if there are several routers under different prefixes.

Example:

const Koa = require('koa')
const Router = require('koa-better-router')

const router1 = new Router({
	prefix: '/api/v1',
	notFound: (ctx, next) => {
		ctx.status = 404
		next()
	}
})
router1.addRoute('GET /foo', ctx => {
	ctx.body = 'foo1'
})

const router2 = new Router({prefix: '/api/v2'})
router2.addRoute('GET /foo', ctx => {
	ctx.body = 'foo2'
})

const app = new Koa()
app.use(router1.middleware())
app.use(router2.middleware())
app.listen(3000)

Then call it:

$ curl localhost:3000/api/v2/foo -v
> GET /api/v2/foo HTTP/1.1
> Host: localhost:3000
> User-Agent: curl/7.54.0
> Accept: */*
>
< HTTP/1.1 404 Not Found
< Content-Type: text/plain; charset=utf-8
< Content-Length: 4
< Date: Thu, 03 Aug 2017 06:33:21 GMT
< Connection: keep-alive
<
foo2

Expected reply: HTTP 200 OK.

[IlyaSemenov]

Funnily enough, if I use an async handler for v1's notFound, the behaviour changes and it doesn't even reach the v2 handler:

        // ...
	notFound: async ctx => {
		ctx.status = 404
		ctx.body = {error: 404}
	}

Response:

$ curl localhost:3000/api/v2/foo -v
...
< HTTP/1.1 404 Not Found
< Content-Type: application/json; charset=utf-8
< Content-Length: 13
< Date: Thu, 03 Aug 2017 07:55:35 GMT
< Connection: keep-alive
<
{"error":404}

(I set body here to distinguish between Koa's default 404 and notFound handler - but it's not what triggers the problem).

So, alas, it's all broken in different ways.