Fix inclusion of url_helpers module in concern

Follow-up to rails#46530.

The dynamically generated `url_helpers` module is an
`ActiveSupport::Concern`.  Therefore, when it is included directly in
another `ActiveSupport::Concern`, its `included` block is deferred until
the latter concern is itself included elsewhere.  Thus, in that case,
the call to `base._routes` in `def self.included(base)` will raise
`NoMethodError` because the `included` block will not yet have defined
the `_routes` method.

This commit prevents the error by first checking if `base` responds to
`_routes`.

Author: jonathanhefner

actionpack/lib/action_dispatch/routing/route_set.rb

@@ -604,7 +604,7 @@ def url_options; {}; end
           # `included` block is run only for the initial inclusion of each copy.
           def self.included(base)
             super
-            if !base._routes.equal?(@_proxy._routes)
+            if base.respond_to?(:_routes) && !base._routes.equal?(@_proxy._routes)
               @dup_for_reinclude ||= self.dup
               base.include @dup_for_reinclude
             end

actionpack/test/dispatch/routing/custom_url_helpers_test.rb

@@ -301,6 +301,17 @@ def test_resolve_urls
     assert_equal "http://www.example.com/manufacturers/apple", Routes.url_helpers.polymorphic_url(@manufacturer)
   end
 
+  def test_url_helpers_module_can_be_included_directly_in_an_active_support_concern
+    concern = Module.new do
+      extend ActiveSupport::Concern
+      include Routes.url_helpers
+    end
+
+    concerned = Class.new { include concern }.new
+
+    assert_equal "http://www.example.com/", concerned.root_url
+  end
+
   def test_defining_direct_inside_a_scope_raises_runtime_error
     routes = ActionDispatch::Routing::RouteSet.new