If the client sends an Expect: header that we cannot meet, we should return a 417 error. E.g.
since we can't meet this expectation, we should return a 417, but instead we forward this to the origin, and return with whatever it responds with.
(Originally found using Coadvisor).
For reference, this is the RFC section: https://tools.ietf.org/html/rfc2616#section-14.20
This is the appropriate section in the HTTPbis RFC: https://tools.ietf.org/html/draft-ietf-httpbis-p2-semantics-22#section-5.1.1