add require for active_support/json to fix #1656

Author: andreaseger

lib/active_model_serializers.rb

@@ -2,6 +2,7 @@
 require 'active_support'
 require 'active_support/core_ext/object/with_options'
 require 'active_support/core_ext/string/inflections'
+require 'active_support/json'
 module ActiveModelSerializers
   extend ActiveSupport::Autoload
   autoload :Model

test/test_helper.rb

@@ -14,7 +14,6 @@
 require 'action_controller'
 require 'action_controller/test_case'
 require 'action_controller/railtie'
-require 'active_support/json'
 require 'active_model_serializers'
 require 'fileutils'
 FileUtils.mkdir_p(File.expand_path('../../tmp/cache', __FILE__))