response_body is an Array in 1.9, so an Array was being pushed onto the cache

actionpack/lib/action_controller/caching/actions.rb

@@ -92,6 +92,8 @@ def _save_fragment(name, options)
         return unless caching_allowed?
 
         content = response_body
+        content = content.join if content.is_a?(Array)
+
         write_fragment(name, content, options)
       end