res.setHeader is not a function
Hi,

```javascript
const Koa = require('koa')
const helmet = require('helmet')
const app = new Koa()

app.use(helmet())

app.use((ctx) => {
  ctx.body = "Hello World"
});

app.listen(3000, () => {
  console.log('server up and running')
})
```
Generates "TypeError: res.setHeader is not a function"

If I comment out "app.use(helmet())" everything works normally.

Thanks