Thread Safety

Use "Thread.current[:included_by]" instead of  the Class variable,
"@@included_by".

The former will work on some newfangled multi-threaded Webserver stack,
which uses separate threads to handle multiple simlutaneous requests
(one request/thread). Dunno that the rest of the application is
thread-safe, but using a class variable, in this context, probably isn't.

Thanks to Sam Ruby for the suggestion.