Sync with latest Instiki trunk. Changes:

1) Upgrade Rails to 1.2.3
2) Revert RedCloth to previous version (who %#$@ cares?)
3) Preserve the Rails Security fix  to vendor/rails/actionpack/lib/action_controller/caching.rb from Revision 80.

vendor/rails/actionpack/lib/action_controller/base.rb

@@ -539,7 +539,7 @@ module ActionController #:nodoc:
       end
 
       def session_enabled?
-        request.session_options[:disabled] != false
+        request.session_options && request.session_options[:disabled] != false
       end
 
     protected