Eliminate warning by initializing nil formats

actionpack/lib/abstract_controller/rendering_controller.rb

@@ -12,6 +12,12 @@ module AbstractController
       self._view_paths ||= ActionView::PathSet.new
     end
 
+    # Initialize controller with nil formats.
+    def initialize(*) #:nodoc:
+      @_formats = nil
+      super
+    end
+
     # An instance of a view class. The default view class is ActionView::Base
     #
     # The view class must have the following methods: