Need a way to disable Vary: Accept-Encoding checking so a plugin can take care of that

I was using the "proxy.config.http.cache.ignore_accept_encoding_mismatch" config setting and providing alt selection hooks to try to let a plugin take over handling of gzip vs uncompressed content, but I found that the checking of Vary: Accept-Encoding headers happened before alt selection, and so the plugin never got the chance to say "gzip content is fine, I will transform it before sending it". So I added additional meaning to that setting to fix that. Perhaps the code should be attached to an all new config setting, but I found the existing meaning of that setting to not do enough.