fix uninitialized instance variable warning

Ruby warns that `@only` and `@except` are not initialized because if they are not passed in, they are never assigned.

Remove circular require in array_serializer

lib/active_model/array_serializer.rb

@@ -1,6 +1,5 @@
 require 'active_model/default_serializer'
 require 'active_model/serializable'
-require 'active_model/serializer'
 
 module ActiveModel
   class ArraySerializer