Hack builder to look for fast_xs instead of insisting on its own String#to_xs

activesupport/lib/active_support/vendor/builder-2.1.2/builder/xchar.rb

@@ -18,7 +18,6 @@ def self.check_for_name_collision(klass, method_name, defined_constant=nil)
 end
 
 if ! defined?(Builder::XChar)
-  Builder.check_for_name_collision(String, "to_xs")
   Builder.check_for_name_collision(Fixnum, "xchr")
 end
 
@@ -105,11 +104,12 @@ def xchr
 # Enhance the String class with a XML escaped character version of
 # to_s.
 #
+require 'active_support/core_ext/string/xchar'
 class String
   # XML escaped version of to_s
   def to_xs
     unpack('U*').map {|n| n.xchr}.join # ASCII, UTF-8
   rescue
     unpack('C*').map {|n| n.xchr}.join # ISO-8859-1, WIN-1252
-  end
+  end unless method_defined?(:to_xs)
 end

activesupport/test/core_ext/hash_ext_test.rb

 require 'abstract_unit'
+require 'builder'
 
 class HashExtTest < Test::Unit::TestCase
   def setup