PATCH: vary on gzip

[Federico Schwindt]

Hi,

I've been pondering on this for a while and I finally convinced myself that
Varnish is doing it wrong.  If http_gzip_support is enabled and the object
is gzip'd we must add a Vary on Accept-Encoding on delivery to keep
intermediary caches happy.

This is done before vcl_deliver is called so it's possible to remove it
before delivering although I can't see why anyone might want to do this.

Alternatively we could add a parameter, e.g. http_gzip_vary, to controls
this but I'm not in favour of adding more knobs. ymmv.

Comments? OKs?

FWIW, nginx support this via gzip_vary (1). It's also mentioned in Google's
Optimizing caching docs (2) and in maxcdn blog post (3).

f.-

1. http://nginx.org/en/docs/http/ngx_http_gzip_module.html#gzip_vary
2. https://developers.google.com/speed/docs/best-practices/caching
3. http://blog.maxcdn.com/accept-encoding-its-vary-important/
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