rails/rails — pull request #8640 by jasl (2012-12-28 14:35:23+00:00)
Title: fix block.arity will raise nil error

```content_tag_for``` will raise ```nil:NilClass``` when not given a block, cause ```content_tag_for_single_record``` will check ```block.arity == 0``` but in this case "block" should be ```nil``` obviously.