ResourceController: Allow overriding routes proxy

Within Solidus, we always want to use the `spree` routes proxy as that
is where all routes are defined. However, many people use the Resource
controller as a parent controller in their main app, and because of this
they need to override all the URL generation methods in the Resource
Controller.

This allows one to specify the routes proxy in a custom controller as
follows:

```rb
module MyApp
  class WidgetsController < Spree::Admin::ResourceController
  private

  def routes_proxy
    main_app
  end
end
```

Engine Developers can use this for engines, as well:

```rb
module MyEngine
  class WidgetsController < Spree::Admin::ResourceController
  private

  def routes_proxy
    my_engine
  end
end
```

backend/app/controllers/spree/admin/resource_controller.rb

@@ -193,7 +193,7 @@ def parent
                   .find_by!(self.class.parent_data[:find_by] => params["#{parent_model_name}_id"])
     instance_variable_set("@#{parent_model_name}", @parent)
   rescue ActiveRecord::RecordNotFound => e
-    resource_not_found(flash_class: e.model.constantize, redirect_url: spree.polymorphic_url([:admin, parent_model_name.pluralize.to_sym]))
+    resource_not_found(flash_class: e.model.constantize, redirect_url: routes_proxy.polymorphic_url([:admin, parent_model_name.pluralize.to_sym]))
   end
 
   def parent?
@@ -238,38 +238,42 @@ def location_after_save
 
   def new_object_url(options = {})
     if parent?
-      spree.new_polymorphic_url([:admin, parent, model_class], options)
+      routes_proxy.new_polymorphic_url([:admin, parent, model_class], options)
     else
-      spree.new_polymorphic_url([:admin, model_class], options)
+      routes_proxy.new_polymorphic_url([:admin, model_class], options)
     end
   end
 
   def edit_object_url(object, options = {})
     if parent?
-      spree.polymorphic_url([:edit, :admin, parent, object], options)
+      routes_proxy.polymorphic_url([:edit, :admin, parent, object], options)
     else
-      spree.polymorphic_url([:edit, :admin, object], options)
+      routes_proxy.polymorphic_url([:edit, :admin, object], options)
     end
   end
 
   def object_url(object = nil, options = {})
     target = object ? object : @object
 
     if parent?
-      spree.polymorphic_url([:admin, parent, target], options)
+      routes_proxy.polymorphic_url([:admin, parent, target], options)
     else
-      spree.polymorphic_url([:admin, target], options)
+      routes_proxy.polymorphic_url([:admin, target], options)
     end
   end
 
   def collection_url(options = {})
     if parent?
-      spree.polymorphic_url([:admin, parent, model_class], options)
+      routes_proxy.polymorphic_url([:admin, parent, model_class], options)
     else
-      spree.polymorphic_url([:admin, model_class], options)
+      routes_proxy.polymorphic_url([:admin, model_class], options)
     end
   end
 
+  def routes_proxy
+    spree
+  end
+
   # Allow all attributes to be updatable.
   #
   # Other controllers can, should, override it to set custom logic

backend/spec/controllers/spree/admin/resource_controller_spec.rb

@@ -2,23 +2,34 @@
 
 require 'spec_helper'
 
-module Spree
+module MyEngine
   module Admin
     class WidgetsController < Spree::Admin::ResourceController
       prepend_view_path('spec/test_views')
 
       def model_class
         Widget
       end
+
+      def routes_proxy
+        my_engine
+      end
     end
   end
 end
 
-describe Spree::Admin::WidgetsController, type: :controller do
+describe MyEngine::Admin::WidgetsController, type: :controller do
   stub_authorization!
 
   # RESOURCE FIXTURE
   before(:all) do
+    # Engine
+    module MyEngine
+      class Engine < Rails::Engine
+        isolate_namespace MyEngine
+      end
+    end
+
     # Database
     class CreateWidgets < ActiveRecord::Migration[5.1]
       def change
@@ -46,13 +57,17 @@ def check_destroy_constraints
     end
 
     # Routes
-    Spree::Core::Engine.routes.draw do
+    MyEngine::Engine.routes.draw do
       namespace :admin do
         resources :widgets do
           post :update_positions, on: :member
         end
       end
     end
+
+    DummyApp::Application.routes.draw do
+      mount MyEngine::Engine, at: "/"
+    end
   end
 
   after(:all) do
@@ -64,12 +79,18 @@ def check_destroy_constraints
     Object.send(:remove_const, :Widget)
 
     # Controller
-    Spree::Admin.send(:remove_const, :WidgetsController)
+    MyEngine::Admin.send(:remove_const, :WidgetsController)
+
+    #Engine
+    MyEngine.send(:remove_const, :Engine)
+    Object.send(:remove_const, :MyEngine)
 
     # Routes
     Rails.application.reload_routes!
   end
 
+  routes { MyEngine::Engine.routes }
+
   describe '#new' do
     subject do
       get :new