Add Support for Variant ETags

[Jeff Kaufman]

On Fri, Dec 13, 2013 at 6:12 AM, Maxim Dounin <mdounin at mdounin.ru> wrote:
> gzipping may result in many different byte representations of
> a resource.  Strict entity tags aren't allowed as a result.
>

Is gzip deterministic, where you only get different byte
representations with different gzip settings?  If so we could include
the gzip settings (or a hash of them) in the etag.