Koa always set wrong `Content-Type` and "Http Status code".
node -v
v9.2.1

npm -v
5.6.0

koajs
2.5.0
----





i have write a function for modify phone number.
but i find Koa always set  wrong `Content-Type` and "Http Status code".

```code
const Router = require('koa-router');
const Response = require('../logic/response');
const Code = require('../logic/code');
const passport = require('../middlewares/snowdream-passport');
const auth = require('../middlewares/snowdream-auth');
const validutil = require('../tools/validutil');

const router = new Router({
    prefix: '/user'
});

//修改手机号码
router.post("/mobile/modify",auth, async (ctx, next) => {
    let omobile = ctx.request.body.omobile;
    let nmobile = ctx.request.body.nmobile;
    let user = ctx.session.passport.user;
    let mobile = user.mobile;
    let User = ctx.model('User');


    // ctx.status = 200;
    ctx.type = 'json';

    if (!(omobile) || !(nmobile) || !(mobile) ||
        !validutil.isValidMobile(omobile) || !validutil.isValidMobile(nmobile)) {

        ctx.body = new Response(Code.INVALID_MOBILE_OR_PASSWORD);

    } else if ((omobile != mobile) ||(omobile == nmobile)){
        ctx.body = new Response(Code.PARAMETERS_ERROR);
    }else {
        console.log("content-type:0 "+ ctx.type)
        console.log("http-status:0 "+ ctx.status)
        user.mobile = nmobile;
        let _user = await User.findOneAndUpdateByMobile(omobile,nmobile).exec();

        if (_user) {
            ctx.body = new Response(Code.SUCCESS);
        } else {
            ctx.body = new Response(Code.FAILED);}
        console.log("content-type:1 "+ ctx.type)
        console.log("http-status:1 "+ ctx.status)
    }
});


module.exports = router;
```

here is my request history:
```
* Preparing request to http://localhost:3000/user/mobile/modify
* Using libcurl/7.54.0 LibreSSL/2.0.20 zlib/1.2.11 nghttp2/1.24.0
* Enable automatic URL encoding
* Disable SSL validation
* Enable cookie sending with jar of 2 cookies
* Found bundle for host localhost: 0x7ff411757980 [can pipeline]
* Re-using existing connection! (#516) with host localhost
* Connected to localhost (::1) port 3000 (#516)
> POST /user/mobile/modify HTTP/1.1
> Host: localhost:3000
> User-Agent: insomnia/5.14.9
> Cookie: ksessionid=94afe4df65c9ea27d0097d1e3031a6415249c79c18783705; ksessionid.sig=_7P8m9bO_FpvDUbi612OuLk_z94
> Accept: application/json; charset=utf-8
> Content-Type: application/x-www-form-urlencoded
> Content-Length: 39
| omobile=17110176639&nmobile=15013196639
* upload completely sent off: 39 out of 39 bytes


< HTTP/1.1 404 Not Found
< Vary: Accept-Encoding
< Content-Type: text/plain; charset=utf-8
* Replaced cookie ksessionid="94afe4df65c9ea27d0097d1e3031a6415249c79c18783705" for domain localhost, path /, expire 1523169444
< Set-Cookie: ksessionid=94afe4df65c9ea27d0097d1e3031a6415249c79c18783705; path=/; expires=Sun, 08 Apr 2018 06:37:24 GMT; httponly
* Replaced cookie ksessionid.sig="_7P8m9bO_FpvDUbi612OuLk_z94" for domain localhost, path /, expire 1523169444
< Set-Cookie: ksessionid.sig=_7P8m9bO_FpvDUbi612OuLk_z94; path=/; expires=Sun, 08 Apr 2018 06:37:24 GMT; httponly
< X-Response-Time: 14ms
< Content-Length: 9
< Date: Sat, 07 Apr 2018 06:37:24 GMT
< Connection: keep-alive

* Received 9 B chunk
* Connection #516 to host localhost left intact
* Saved 2 cookies
```
here is my console log:
```
/Users/snowdream/.nvm/versions/node/v9.2.1/bin/node /Users/snowdream/workspace/snowdream/startup/truck/server/truck/app.js DEBUG
(node:75559) DeprecationWarning: `open()` is deprecated in mongoose >= 4.11.0, use `openUri()` instead, or set the `useMongoClient` option if using `connect()` or `createConnection()`. See http://mongoosejs.com/docs/4.x/docs/connections.html#use-mongo-client
[demo] start-quick is starting
content-type:0 application/json
http-status:0 404
content-type:1 text/plain
http-status:1 404
```

### My question is:
1. i have set "Accept: application/json; charset=utf-8",but why response give the content-type "text/plain; charset=utf-8"?
2. i have set the result to the ctx.body, but why i get the status code 404?