Call Response::exitCallback on all setHeaders that should end early

[g105b]

The only header that should end early is Location as far as I can tell, but maybe there are more. Need to research before implementing.

[g105b]

In WebEngine's RequestHandler::forceTrailingSlashes() function, the Location header is set manually, along with the status code. This should prevent the request handler from executing any further, by calling the exit callback.