[Reviewer Andy] Fix anti-looping in SproutletProxy so AIO node works

Andy

Can you review this fix for the AIO nodes, as per email discussion this morning.  I've updated the UTs to test this new behaviour, but I haven't run the AIO node yet - I was going to do this as part of re-cutting the release.

Mike
The old test no longer works - what now happens is that the fwd Sproutlet does route the request externally, but it loops back to the SproutletProxy which removes the unrecognised Route header, tries to forward the request to the RequestURI and fails.