Work around strange Ruby 1.9 autoload issue by using absolute load paths for...

Work around strange Ruby 1.9 autoload issue by using absolute load paths for tests (for Active Model too)

activemodel/test/test_helper.rb

@@ -7,7 +7,7 @@
 require 'active_model'
 require 'active_model/state_machine'
 
-$:.unshift File.dirname(__FILE__) + "/../../activesupport/lib"
+$:.unshift File.expand_path('../../../activesupport/lib', __FILE__)
 require 'active_support'
 require 'active_support/test_case'