Make Rake::DSL methods available within Railtie rake_tasks block

Fixes incompatibility introduced by Rake 0.9.0

railties/lib/rails/application.rb

@@ -185,12 +185,14 @@ module Rails
     end
 
     def initialize_tasks
+      self.class.rake_tasks do
         require "rails/tasks"
         task :environment do
           $rails_rake_task = true
           require_environment!
         end
       end
+    end
 
     def initialize_generators
       require "rails/generators"

railties/lib/rails/railtie.rb

@@ -178,6 +178,7 @@ module Rails
     end
 
     def load_tasks
+      extend Rake::DSL if defined? Rake::DSL
       self.class.rake_tasks.each(&:call)
 
       # load also tasks from all superclasses