Create DefaultSerializer so that as_json uses same interface.

This is to ensure that PORO's as_json is called if no serializer
is specified.

Original behaviour was that serializable_hash was being called,
overriding the as_json method.

lib/active_model/array_serializer.rb

@@ -1,6 +1,15 @@
 require "active_support/core_ext/class/attribute"
 
 module ActiveModel
+  class DefaultSerializer
+    attr_reader :object
+    def initialize(object)
+      @object = object
+    end
+    def serializable_hash
+      @object.as_json
+    end
+  end
   # Active Model Array Serializer
   #
   # It serializes an Array, checking if each element that implements
@@ -30,7 +39,7 @@ module ActiveModel
         if serializer
           serializer.new(item, @options)
         else
-          item
+          DefaultSerializer.new(item)
         end
       end
     end