[Haml] Fix a bizarre interaction with ActiveModel.

Closes gh-129

doc-src/HAML_CHANGELOG.md

@@ -9,6 +9,8 @@
 
 * Make sure `form_for` blocks are properly indented under Rails 3.0.0.beta.3.
 
+* Don't prevent ActiveModel form elements from having error formatting applied.
+
 ## 2.2.23
 
 [Tagged on GitHub](http://github.com/nex3/haml/commit/2.2.23).

lib/haml/helpers/action_view_mods.rb

@@ -126,7 +126,9 @@ module ActionView
       end
 
       def content_tag(*args)
-        content_tag_with_haml(*args)
+        html_tag = content_tag_with_haml(*args)
+        return error_wrapping(html_tag) if respond_to?(:error_wrapping)
+        return html_tag
       end
     end