Set the 'via' response header instead of 'proxy-agent'
Add a 'x-original-content-length' response header

This contains the original 'content-length' header value which
allows the UA to determine its own total compression ratio.
Optionally (but by default) accumulate in ingress when we don't have content-length

This allows us to then write a content-length (and more importantly,
x-original-content-length).
Merge branch 'header-stuff' into develop