enabling flatten json as default adapter

lib/active_model/serializer/configuration.rb

@@ -6,7 +6,7 @@ module ActiveModel
 
       included do |base|
         base.config.array_serializer = ActiveModel::Serializer::ArraySerializer
-        base.config.adapter = :json
+        base.config.adapter = :flatten_json
       end
     end
   end

test/test_helper.rb

@@ -20,7 +20,6 @@ class Foo < Rails::Application
     config.active_support.test_order = :random
     config.logger = Logger.new(nil)
     ActionController::Base.cache_store = :memory_store
-    ActiveModel::Serializer.config.adapter = :flatten_json
   end
 end
 FileUtils.mkdir_p(File.expand_path('../../tmp/cache', __FILE__))