Nested exposure doesn't work

This code:

``` ruby
class User < Grape::Entity
  expose :photo do |p|
    expose :large do
      p.photo.url(:large)
    end
  end
end
```

Returns:

```
NoMethodError (undefined method `expose' for #<API::User:0x00000023cdec68>):
```

grape-entity: v0.4.8
grape: v0.13.0
