Merge pull request activeadmin#27 from matekb/master

Fix for issue activeadmin#25

Author: elia

lib/active_admin/mongoid/helpers/collection.rb

@@ -2,7 +2,7 @@ module ActiveAdmin
   module Helpers
     module Collection
       def collection_size(collection=collection)
-        collection.count(true)
+        collection.count()
       end
     end
   end