Prefix seems to be broken
ubuntu 16.04
Node.js 10.15.1
koa 2.7.0
koa-joi-router 5.2.0

```js
'use strict'

const Koa = require('koa')
const app = new Koa()

const router = require('koa-joi-router')

const orders = router()
orders.get('/', ctx => {
  ctx.body = 'Orders response'
})

orders.prefix('orders')

app.use(orders.middleware())

app.listen(3000)
```

### Actual behavior:
`localhost:3000/orders` returns 404 Not Found

### Expected behavior:
`localhost:3000/orders` returns 200 'Orders response`

Connected issue created in [koa-router](https://github.com/ZijianHe/koa-router/issues/496)