I've come across situations in the past where a resource couldn't be gzipped. My original goal was to have another function, like `gzip.AllExcept` or `gzip.Except`, that would take a map of non-gzippable resources and return `gzip.serveGzip` for resources not in that map.  

If not needed, I can refactor things to rename `gzip.All` and move the functionality from `gzip.serveGzip` to the refactored function.