[varnish] normalize Accept-Encoding header

default.vcl

# normalize Accept-Encoding header
if (req.http.Accept-Encoding) {
        if (req.url ~ "\.(jpeg|jpg|png|gif|gz|tgz|bz2|tbz|zip|flv|pdf|mp3|ogg)$") {
                remove req.http.Accept-Encoding; # already compressed
        }
        elsif (req.http.Accept-Encoding ~ "gzip") {
                set req.http.Accept-Encoding = "gzip";
        }
        elsif (req.http.Accept-Encoding ~ "deflate") {
                set req.http.Accept-Encoding = "deflate";
        }
        else {
                remove req.http.Accept-Encoding;
        }
}