Fetch namespaced model in the namespaced controller firstly

I would like to make controller to look for model in namespace firstly when controller is namespaced.

Assume we have model in the module:

```ruby
module Admin
  class User < ActiveRecord::Base
    belongs_to :group
  end
end
```

which belongs to group:

```ruby
module Admin
  class Group < ActiveRecord::Base
    has_many :users
  end
end
```

I want to have controller with nested resource users for group:

```ruby
module Admin
  class Users < InheritedResources::Base
    belongs_to :group
  end
end
```

Inherited resources doesn't find parent for users collection as expected because it doesn't try too look up it in the `Admin` module. It tries to find only `Group` model which is not exist.

This fix solves problem with getting model in the namespace firstly, and if it doesn't find it it tries to get model without namespace.

Note: it also ready to find class model in the nested space, for example `A::B::MyModel`.