Don't used defined? on a scoped constant since it results in a const_missing call.

git-svn-id: http://svn-commit.rubyonrails.org/rails/trunk@3298 5ecf4fe2-1ee6-0310-87b1-e25e094e27de

actionpack/lib/action_controller/caching.rb

@@ -517,7 +517,7 @@ module ActionController #:nodoc:
       end
     end
 
-    if defined?(ActiveRecord::Observer)
+    if defined?(ActiveRecord) and defined?(ActiveRecord::Observer)
       class Sweeper < ActiveRecord::Observer #:nodoc:
         attr_accessor :controller