Make response header copying compliant with RFC 2616
Make response header copying compliant with RFC 2616

RFC 2616 (see https://www.w3.org/Protocols/rfc2616/rfc2616-sec13.html)
explicitly specifies which headers should be filtered by proxies and
which ones should be transmitted unchanged.

Prior to this change, we just copied all response headers
through. With this change we switch over to only copying the
end-to-end headers and filtering out the hop-by-hop headers.

google/inverting-proxy