Adding missing required statement

[#5056 state:resolved]
Signed-off-by: NSantiago Pastorino <santiago@wyeworks.com>

activemodel/test/cases/serializeration/json_serialization_test.rb

 require 'cases/helper'
 require 'models/contact'
+require 'models/automobile'
 require 'active_support/core_ext/object/instance_variables'
 
 class Contact