headers: strip accept-bytes and vary-accept-encoding

If we're proxying the origin server might advertise `Accept-Ranges: bytes` but
we don't support range requests.  So we need to remove the header.  We also need
to not add `Vary: Accept-Encoding` twice, and because the gzip encoding will add
it later we need to remove it here.
Merge pull request #132 from pagespeed/jefftk-strip-headers

headers: strip accept-bytes and vary-accept-encoding