adding json_api 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 = :json_api
       end
     end
   end

test/fixtures/poro.rb

@@ -1,3 +1,5 @@
+ActiveModel::Serializer.config.adapter = :json
+
 class Model
   def initialize(hash={})
     @attributes = hash