Upgrading to 0.4.6 breaks Runtime Exposure

Here is my entity:

```
expose :my_custom_field

private

def my_custom_field
  1
end
```

After upgrading to 0.4.6, I started to get the following error: 

```
NoMethodError (MyEntity missing attribute my_custom_field on #<MyEntity:0xdc60c70>):
```

If I downgrade to `0.4.5` everything works fine.

Thanks.
