Prevent calls to `scoped` on decorated associations

Calling `parent.children.decorate` breaks `inverse_of` support, due to the call to `scoped` in `Decoratable.decorate`.

This fix changes decorated associations to do something equivalent to 

```ruby
parent.children.decorator_class.decorate(parent.children)
```

which does the same thing as `parent.children.decorate` but without calling `scoped`, and thus preserves the `inverse_of` link.

Closes #412.