Add support for specifying the serializer for an association as a String.

This enables the deferral of the resolution of the serializer class to prevent NameError exceptions due to reference cycles between serializer classes.

lib/active_model/serializer/associations.rb

@@ -39,7 +39,8 @@ module ActiveModel
         end
 
         def target_serializer
-          option(:serializer)
+          serializer = option(:serializer)
+          serializer.is_a?(String) ? serializer.constantize : serializer
         end
 
         def source_serializer