Fix authenticated engine routes (#4081)

Fix infinite loop in authenticated engine routes in Rails 5

https://github.com/plataformatec/devise/issues/3705

test/integration/mounted_engine_test.rb

@@ -0,0 +1,36 @@
+require 'test_helper'
+
+class MyMountableEngine
+  def self.call(env)
+    ['200', { 'Content-Type' => 'text/html' }, ['Rendered content of MyMountableEngine']]
+  end
+end
+
+# If disable_clear_and_finalize is set to true, Rails will not clear other routes when calling
+# again the draw method. Look at the source code at:
+# http://www.rubydoc.info/docs/rails/ActionDispatch/Routing/RouteSet:draw
+Rails.application.routes.disable_clear_and_finalize = true
+
+Rails.application.routes.draw do
+  authenticate(:user) do
+    mount MyMountableEngine, at: '/mountable_engine'
+  end
+end
+
+class AuthenticatedMountedEngineTest < Devise::IntegrationTest
+  test 'redirects to the sign in page when not authenticated' do
+    get '/mountable_engine'
+    follow_redirect!
+
+    assert_response :ok
+    assert_contain 'You need to sign in or sign up before continuing.'
+  end
+
+  test 'renders the mounted engine when authenticated' do
+    sign_in_as_user
+    get '/mountable_engine'
+
+    assert_response :success
+    assert_contain 'Rendered content of MyMountableEngine'
+  end
+end