fix belongs_to for models without namespace in namespaced controllers

Bug was introduced by this commit https://github.com/josevalim/inherited_resources/commit/630a2775fab351540fc9bd8aa0ce43d4d039f44a, and brake inherited_resources in such situation:

    class Group < ActiveRecord::Base
    end 
    class User < ActiveRecord::Base
    end 
    module Admin
      class Users < InheritedResources::Base
        belongs_to :group
      end
    end