`proxyRes` event, provide access to the req and res objects

Further to the pull request by @Domiii - #635 - this pull request adds the res object to the proxyRes event as well.

Also adding a test for the proxyRes event.

I wasn't about the specifics of updating other people's pull request, so just thought a new one might be a good option...
Apologies if that's less ideal :-s