Use router_name from scope if available

Devise.available_router_name currently returns either
Devise.router_name or :main_app. As such, any redirecting is done
within either of those contexts. Which leads to undesirable redirects
for scopes that reside in an isolate_namespace mounted engine.

This commit makes it possible for FailureApp’s redirect behavior to be
performed in the context of the router_name given to devise_for.

Test case added to cover undesirable behavior. Without change to
lib/devise/failure_app.rb, test case throws exception.

lib/devise/failure_app.rb

@@ -120,7 +120,8 @@ module Devise
         opts[:script_name] = config.relative_url_root
       end
 
-      context = send(Devise.available_router_name)
+      router_name = Devise.mappings[scope].router_name || Devise.available_router_name
+      context = send(router_name)
 
       if context.respond_to?(route)
         context.send(route, opts)

test/failure_app_test.rb

@@ -26,6 +26,22 @@ class FailureTest < ActiveSupport::TestCase
     end
   end
 
+  class FakeEngineApp < Devise::FailureApp
+    class FakeEngine
+      def new_user_on_engine_session_url _
+        '/user_on_engines/sign_in'
+      end
+    end
+
+    def main_app
+      raise 'main_app router called instead of fake_engine'
+    end
+
+    def fake_engine
+      @fake_engine ||= FakeEngine.new
+    end
+  end
+
   def self.context(name, &block)
     instance_eval(&block)
   end
@@ -85,6 +101,13 @@ class FailureTest < ActiveSupport::TestCase
       end
     end
 
+    test 'returns to the default redirect location considering the router for supplied scope' do
+      call_failure app: FakeEngineApp, 'warden.options' => { scope: :user_on_engine }
+      assert_equal 302, @response.first
+      assert_equal 'You need to sign in or sign up before continuing.', @request.flash[:alert]
+      assert_equal 'http://test.host/user_on_engines/sign_in', @response.second['Location']
+    end
+
     if Rails.application.config.respond_to?(:relative_url_root)
       test 'returns to the default redirect location considering the relative url root' do
         swap Rails.application.config, relative_url_root: "/sample" do