Send get method some cases return Not Found
I try to write an simple jwt api with mongodb, the `/user` router code like this:

``` js
router.use('/user', async (ctx, next) => {
  // do sth. and run next
}, async (ctx) => {
  try {
    // here do some mongodb query, and get right result
    ctx.body = {
      // I want to return query result here, but return 'Not Found' and 404
    }
  } catch (e) {}
})
```

return 'Not Found' and 404 in postman

But I write a simple `/test` router code:

``` js
router.get('/test', async (ctx, next) => {
  ctx.body = "456"
  await next()
}, async (ctx) => {
  ctx.body += "1234"
})
```

I can get "4561234" in postman. Confused.


---

``` json
{
  "dependencies": {
    "crypto-js": "^4.0.0",
    "jsonwebtoken": "^8.5.1",
    "koa": "^2.13.1",
    "koa-bodyparser": "^4.3.0",
    "koa-router": "^10.0.0",
    "mongoose": "^5.12.13"
  }
}
```

Tks for reading my poor English