Force _routes to be redefined on re-inclusion

This fixes an regression where _routes were set incorrectly when the
inheritance chain went from one route namespace (isolated engine or main
app) to another and then back to the original. Because the url_helpers
module was being cached and was being re-included this was not setting
_routes.

This commit solves the issue by detecting that case and redefining
_routes in that case. We could have always performed the redefinition,
but it's a pretty uncommon case, so we might as well only do it when
necessary.

actionpack/lib/abstract_controller/railties/routes_helpers.rb

@@ -7,11 +7,27 @@ module AbstractController
         Module.new do
           define_method(:inherited) do |klass|
             super(klass)
-            if namespace = klass.module_parents.detect { |m| m.respond_to?(:railtie_routes_url_helpers) }
+
+            namespace = klass.module_parents.detect { |m| m.respond_to?(:railtie_routes_url_helpers) }
+            actual_routes = namespace ? namespace.railtie_routes_url_helpers._routes : routes
+
+            if namespace
               klass.include(namespace.railtie_routes_url_helpers(include_path_helpers))
             else
               klass.include(routes.url_helpers(include_path_helpers))
             end
+
+            # In the case that we have ex.
+            #   class A::Foo < ApplicationController
+            #   class Bar < A::Foo
+            # We will need to redefine _routes because it will not be correct
+            # via inheritance.
+            unless klass._routes.equal?(actual_routes)
+              klass.redefine_singleton_method(:_routes) { actual_routes }
+              klass.include(Module.new do
+                define_method(:_routes) { @_routes || actual_routes }
+              end)
+            end
           end
         end
       end