Cleanup for rb 2.2

Author: boie0025

lib/active_admin/mongoid/helpers/collection.rb

@@ -5,7 +5,8 @@ module Collection
       alias original_collection_size collection_size
       original_collection_size = instance_method(:collection_size)
 
-      def collection_size(collection=collection)
+      def collection_size(collection = nil)
+        collection ||= self.collection
         if collection.is_a?(::Mongoid::Criteria)
           collection.count(true)
         else