ctx.request.origin is wrong
Ciao!

How are you?

Weird output:
```js
    app.use(ctx => {
        ctx.body = {
            hello: 'world',
            origin: ctx.request.origin,
        }
    })
```

![image](https://user-images.githubusercontent.com/28588124/38821809-e390f5e6-41a1-11e8-9420-353f36c67142.png)

![image](https://user-images.githubusercontent.com/28588124/38821842-f3aa0b84-41a1-11e8-80b8-52e61e64070d.png)
