load_generators from engine should also handle self automatically.

railties/lib/rails/railtie.rb

@@ -174,11 +174,11 @@ def config
     def eager_load!
     end
 
-    def load_console(app)
+    def load_console(app=self)
       self.class.console.each { |block| block.call(app) }
     end
 
-    def load_tasks(app)
+    def load_tasks(app=self)
       extend Rake::DSL if defined? Rake::DSL
       self.class.rake_tasks.each { |block| block.call(app) }
 
@@ -190,7 +190,7 @@ def load_tasks(app)
       end
     end
 
-    def load_generators(app)
+    def load_generators(app=self)
       self.class.generators.each { |block| block.call(app) }
     end