Merge pull request #2466 from Shopify/0-9-stable-const-defined

[0.9 stable] Avoid `Object.constants.include?`

Author: byroot

lib/active_model/serializer.rb

@@ -62,7 +62,7 @@ def serializer_for(resource, options = {})
         if resource.respond_to?(:serializer_class)
           resource.serializer_class
         elsif resource.respond_to?(:to_ary)
-          if Object.constants.include?(:ArraySerializer)
+          if defined?(::ArraySerializer)
             ::ArraySerializer
           else
             ArraySerializer