Check definition on constant not string

git-svn-id: http://svn-commit.rubyonrails.org/rails/trunk@1250 5ecf4fe2-1ee6-0310-87b1-e25e094e27de

actionpack/lib/action_controller/caching.rb

@@ -443,7 +443,7 @@ def cache_sweeper(*sweepers)
       end
     end
     
-    if defined?("ActiveRecord")
+    if defined?(ActiveRecord::Observer)
       class Sweeper < ActiveRecord::Observer
         attr_accessor :controller