Is it possible to specify module in devise_scope in isolated engine

[Simon Iong]

I have a engine, which contain devise, and I know there's a `module` option for `devise_for` to make sure all devise controller reside under the scope of engine itself. Please refer to [wiki page here](https://github.com/plataformatec/devise/wiki/How-To:-Use-devise-inside-a-mountable-engine)

But there seem no `module` options for `devise_scope` while I need to custom my devise operations within engine. Anyway, here's my setting:

route.rb in MyEngine

``` ruby 

  devise_for :admins, class_name: "MyEngine::Admin",

                      module: :devise,

                      skip: [:registrations, :passwords]

  devise_scope :admin do

    get 'admins/edit' => 'devise/registrations#edit', :as => 'edit_admin_registration'

    patch 'admins/:id'  => 'admin/registrations#update', :as => 'admin_registration'

  end

```

route.rb in MainApp

``` ruby 

Rails.application.routes.draw do

  mount MyEngine::Engine => "/"

end

```

and while I check the route in my `MainApp`, I found the route setting:

``` ruby

Routes for MyEngine::Engine:

       new_admin_session GET      /admins/sign_in(.:format)              devise/sessions#new

           admin_session POST     /admins/sign_in(.:format)              devise/sessions#create

   destroy_admin_session DELETE   /admins/sign_out(.:format)             devise/sessions#destroy

            admin_unlock POST     /admins/unlock(.:format)               devise/unlocks#create

        new_admin_unlock GET      /admins/unlock/new(.:format)           devise/unlocks#new

                         GET      /admins/unlock(.:format)               devise/unlocks#show

 edit_admin_registration GET      /admins/edit(.:format)                 my_engine/devise/registrations#edit

      admin_registration PATCH    /admins/:id(.:format)                  my_engine/admin/registrations#update

```

The problem is the last 2 route mappings, what I want is to remove "my_engine" prefix.

``` ruby

 edit_admin_registration GET      /admins/edit(.:format)                 devise/registrations#edit

      admin_registration PATCH    /admins/:id(.:format)                  admin/registrations#update

```

Is it possible to add `module` options in devise_scope, or are there any way to archive this?