Fix AC::Metal#response_body= to store same value on Ruby 1.8 and 1.9

This was because String#respond_to?(:each) differs in 1.8 and 1.9

actionpack/lib/action_controller/metal.rb

@@ -182,7 +182,13 @@ module ActionController
     end
 
     def response_body=(val)
-      body = val.nil? ? nil : (val.respond_to?(:each) ? val : [val])
+      body = if val.is_a?(String)
+        [val]
+      elsif val.nil? || val.respond_to?(:each)
+        val
+      else
+        [val]
+      end
       super body
     end