PATCH: vary on gzip

[Nils Goroll]

As discussed on IRC: I do very much appreciate that you've but this back onto
the agenda, this is important and Varnish needs to get the default Vary right.

But I think this needs to live in fetch (before calling v_b_r): The Vary header
does not depend upon the request, so it does not need to live in deliver. Doing
things at delivery time is more costly than at fetch time (because we probably
cache the result).

When backends handle gzip, we also end up with a cache-object with Vary: A-E (at
least for sane backends), so we should get the same result when Varnish does the
compression.

vry_cmp already skips Accept-Encoding, so we should be fine at lookup time.

Nils