make the routes reader private

nobody should be touching the routes hash without going through the
NamedRouteCollection object.

actionpack/lib/action_dispatch/routing/route_set.rb

@@ -89,6 +89,7 @@ module ActionDispatch
       class NamedRouteCollection
         include Enumerable
         attr_reader :routes, :url_helpers_module, :path_helpers_module
+        private :routes
 
         def initialize
           @routes  = {}

actionpack/test/dispatch/mount_test.rb

@@ -49,7 +49,7 @@ class TestRoutingMount < ActionDispatch::IntegrationTest
   def test_app_name_is_properly_generated_when_engine_is_mounted_in_resources
     assert Router.mounted_helpers.method_defined?(:user_fake_mounted_at_resource),
           "A mounted helper should be defined with a parent's prefix"
-    assert Router.named_routes.routes[:user_fake_mounted_at_resource],
+    assert Router.named_routes.key?(:user_fake_mounted_at_resource),
           "A named route should be defined with a parent's prefix"
   end