Minimal example for double gzip by gorilla/handlers.CompressHandler

The CompressHandler of gorilla/handlers gzips responses twice if they're already gzipped, i.e. it ignores the Content-Encoding header.

How to reproduce

Requirements:

• go 1.12+

$ go run main.go &
$ curl -H 'Accept-Encoding: gzip' localhost:9999 | gunzip
# prints garbage
$ curl -H 'Accept-Encoding: gzip' localhost:9999 | gunzip | gunzip
# works fine

A working implementation is served on localhost:19999.