[Rails 3.2] class in lib does not override class in engine

[Erwin]

I am mounting the engine  'opro_oauth'  gem in my routes

mount_opro_oauth

I want to overwrite a method from the engine lib:     /lib/opro/auth_provider/devise.rb

module Opro
  module AuthProvider
    class Devise
...
      def find_user_for_auth(params)
      ...
      end
    end
  end
end

So, I added the modified devise.rb n my iapp , /lib/opro/auth_provider/devise.rb

and added     config.autoload_paths += ["#{config.root}/lib/opro/auth_provider"]   in my application.rb

I inserted a debigger statement to check if the method is reached but no way, the original method is always run..  what's wrong with my setup... ??

thanks for feedback