Added support for :only and :except methods.

It is possible now to filter returned attributes and associations by:

  UserSerializer.new(user, only: [:first_name, :last_name])
  UserSerializer.new(user, except: :first_name)

lib/active_model/serializer.rb

@@ -363,6 +363,8 @@ module ActiveModel
     end
 
     def include?(name)
+      return false if options.key?(:only) && !Array(options[:only]).include?(name)
+      return false if options.key?(:except) && Array(options[:except]).include?(name)
       send INCLUDE_METHODS[name]
     end