Don't do gzip compression if the URL path ends in .gz or .tgz

Don't do gzip compression if the URL path ends in .gz or .tgz
This confuses Safari and triggers a download of the page,
even though it's pretty clearly labeled as viewable HTML.
Bad Safari! Bad!

Bug is still present in Safari 2.0.4/Mac and 3.0.2/Win.

We have had a live hack for this on Wikimedia sites for .gz files;
hadn't noticed the .tgz problem before though.

While I was in there, also made a tweak so wfGzipHandler() makes
sure the Vary: header is there whether the current agent sent us
an Accept-Encoding header or not.