PLUGWEB-15: add a way to disable sitemesh in a request at the last minute

Sometimes, the decoratormapper setup selects a request for decoration, but it is not until the action executes that it realizes it does not want decoration.

This change allows a request to set a request attribute. If that attribute is set before getOutputStream()/getWriter() is called, then sitemesh will not decorate (nor buffer) the request, even if it would have otherwise.