AssertionError [ERR_ASSERTION]: headers have already been sent
This is my code.
```js
import Koa from 'koa';
import Router from 'koa-router';
import multer from 'koa-multer';

const router = new Router();
const app = new Koa();
const uploadMulter = multer({ dest: 'public/upload/' });

router.post('/upload', uploadMulter.array('files'), async (ctx, next) => {
    ctx.status = 200;
    ctx.body = { answer: 'ok' };
    return next();
});
```
When I upload files, it upload to the dir successfully, but when I set ```ctx.status``` or ```ctx.body``` , the log prints this, and return to client ```404 (Not Found)```
```
(node:7299) UnhandledPromiseRejectionWarning: Unhandled promise rejection (rejection id: 2): AssertionError [ERR_ASSERTION]: headers have already been sent
(node:7299) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.
```