Add default value for request body.

Since i was using some middleware with `koa-body` sometimes request body is undefined and then will cause some error `Cannot read property xx of undefined` with the requset.body. So i was thinking add a default value `{}` is a good idea to solved the issue like this.
If not maybe i need add a middleware like 
```
function *(next) {
     this.request.body = this.request.body || {}
    yield next
}
```