Require 'active_support/core_ext' instead of 'activesupport'.

This makes the tests Rails 3 compatible. Adding /core_ext is necessary
or we don't have access to the class_inheritable_reader method.

test/test_helper.rb

 require 'rubygems'
 require 'test/unit'
-require 'activesupport'
+require 'active_support/core_ext'
 
 $LOAD_PATH.unshift(File.dirname(__FILE__))
 $LOAD_PATH.unshift(File.join(File.dirname(__FILE__), '..', 'lib'))