Fix content_tag helper, only try to mark content as safe if it is not a hash of options

The Padrino content_tag helper can be used in two ways: http://www.padrinorb.com/api/Padrino/Helpers/TagHelpers.html#content_tag-instance_method
The second one (content given in a block not string) failed after this recent commit: a52effc119cdd29304efbef92b766ab79ea0a1fc