Always set a serializer for each item of an Array

model.active_model_serializer could return nil so we need to ensure that
if serializer is not setted we set DefaultSerializer to it.

This reverts commit 64ed05c484.

Fixes #318

lib/active_model/array_serializer.rb

@@ -49,9 +49,8 @@ module ActiveModel
           serializer = options[:each_serializer]
         elsif item.respond_to?(:active_model_serializer)
           serializer = item.active_model_serializer
-        else
-          serializer = DefaultSerializer
         end
+        serializer ||= DefaultSerializer
 
         serializable = serializer.new(item, options.merge(:root => nil))