Receiving 404 "Not Found" but i'm setting ctx.response.body = token
Hi!

I keep getting `404 "Not Found"` while i've already set `ctx.response.body = token` *where `toke` is a valid `token` in my tests (with `supertest` & in `postman`).

Is there a way to publish the token and override the "Not found" message? 

Here is how i'm making the token
``` js
const signUp = async (ctx, next) => {
    
    const bodyInfo = await ctx.request.body;

    if(!bodyInfo.username || !bodyInfo.password) {
        ctx.status = 402;
        ctx.body = "Error, username and password must be provided!";
    }

    const userInst = new User(bodyInfo);

    userInst.save(async(err, user) => {
        
        if(err) { return next(err); }

        const token = tokenForUser(user);

        ctx.body = token; //- <-- Passing the token.

        return next();
    });
};

export { signUp, signIn };
```
the route that i hit to get this middleware is 
``` js
router.post('/signup', signUp);

//- oddly enough

router.post('/signup', signUp, async (ctx, next) => {
    const res = await ctx.response;
    
     //- returns a lot of good stuff including `token` but can't publish token outside.
     console.log(res); 
```

I wish to override "Not Found" message and the whole `ctx.response` cause i get really long weird pre made 404s for a route that is preset.

Here is my stack trace.

```
  1) Authentication signs up:
     Error: expected 200 "OK", got 404 "Not Found"
      at Test._assertStatus (node_modules/supertest/lib/test.js:266:12)
      at Test._assertFunction (node_modules/supertest/lib/test.js:281:11)
      at Test.assert (node_modules/supertest/lib/test.js:171:18)
      at assert (node_modules/supertest/lib/test.js:131:12)
      at node_modules/supertest/lib/test.js:128:5
      at Test.Request.callback (node_modules/superagent/lib/node/index.js:631:3)
      at IncomingMessage.<anonymous> (node_modules/superagent/lib/node/index.js:795:18)
      at endReadableNT (_stream_readable.js:974:12)
      at _combinedTickCallback (internal/process/next_tick.js:74:11)
      at process._tickCallback (internal/process/next_tick.js:98:9)
```

Please help and if i'm missing anything [i asked this question on stackoverflow here](http://stackoverflow.com/questions/41568759/koa-js-returning-404-not-found-after-signup-instead-of-the-token-from-ctx-bod).

Thank you.