Can't pipe stream to ctx.res
I'm trying to pipe streams to my response but it seems didn't work

Simple test case:
```node
const Koa = require('koa');
const app = new Koa();
const Router = require('koa-better-router');
const router = Router().loadMethods();
const request = require('request');

app.use(async (ctx, next) => {
    const start = new Date();
    await next();
    const ms = new Date() - start;
    console.log(`${ctx.method} ${ctx.url} - ${ms}ms`);
});

router.get('/', async (ctx) => {
    ctx.req.pipe(request(`https://www.google.com/?q=${ctx.query.q}`)).pipe(ctx.res);
});

app.use(router.middleware());

app.listen(3000, () => {
    console.log('listening on port 3000');
});

```
An error occurred:
```sh
$ node index.js 
listening on port 3000
GET / - 35ms
internal/streams/legacy.js:59
      throw er; // Unhandled stream error in pipe.
      ^

Error: write after end
    at ServerResponse.write (_http_outgoing.js:450:15)
    at Request.ondata (internal/streams/legacy.js:16:26)
    at emitOne (events.js:96:13)
    at Request.emit (events.js:189:7)
    at IncomingMessage.<anonymous> (/Users/knowlet/test/node_modules/request/request.js:1088:12)
    at emitOne (events.js:96:13)
    at IncomingMessage.emit (events.js:189:7)
    at IncomingMessage.Readable.read (_stream_readable.js:381:10)
    at flow (_stream_readable.js:761:34)
    at resume_ (_stream_readable.js:743:3)
```

And `req.pipe(request(...)).pipe(res)` works in express.js, I'm wondering is there anything I do wrong?

```sh
$ node -v
v7.6.0
```