@Sameer Vohra - here is an example that is actually acting as a proxy. It copies the received request headers and adds the "via" header on the way back out. I don't think it conforms exactly as a proxy should but might be a better example than the tests.
Thanks Rus. I figured I needed to send the body but the requestspec buffer (and the body flatmap) wasn't obvious to me. Works like a charm and I've commented the full solution/output on the Gist for future reference.