unexpected response on stream from stdout
I need to implement an application to stream stdout into browser

``` js
const shell = require('shelljs')
const PassThrough = require('stream').PassThrough

app.use(function* () {

  // in real app, stream generating by another external application
  const command = 'cat ./video.mp4'
  const ch = shell.exec(command, { silent: true, async: true })

  this.set('Content-Disposition', 'attachment')
  this.attachment('video.mp4')

  this.body = ch.stdout.pipe(PassThrough())
})
```

the problem is video size is ~570KB, but downloaded video is ~1MB and can't play that via video players
Is Koa buffer or change the response ? or something else ?
