Add some comments related to Hash method check.

railties/lib/rails/tasks/routes.rake

@@ -3,6 +3,9 @@ task :routes => :environment do
   Rails::Application.reload_routes!
   all_routes = ENV['CONTROLLER'] ? Rails.application.routes.routes.select { |route| route.defaults[:controller] == ENV['CONTROLLER'] } : Rails.application.routes.routes
   routes = all_routes.collect do |route|
+    # TODO: The :index method is deprecated in 1.9 in favor of :key
+    # but we don't have :key in 1.8.7. We can remove this check when
+    # stop supporting 1.8.x
     key_method = Hash.method_defined?('key') ? 'key' : 'index'
     name = Rails.application.routes.named_routes.routes.send(key_method, route).to_s
     reqs = route.requirements.empty? ? "" : route.requirements.inspect