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

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