Fix incompatibility presented with 4.2 code

Update the code that uses the internal rails api to be compatible with rails 4.2.

> ActiveRecord::Base#reflections now returns a hash with string keys instead of symbol keys.

[PR] rails/rails#17718

lib/agile_serializer.rb

@@ -42,7 +42,7 @@ def serialization_options(set)
             hash.merge(class_name)
           else
             begin
-              true_name = reflections[class_name].options[:class_name] || class_name
+              true_name = reflections[class_name.to_s].options[:class_name] || class_name
               klass = true_name.to_s.classify.constantize
               hash[class_name] = klass.serialization_options(set)
               hash