Honor Accept-Encoding header in gzip transcoder

This avoids sending compressed data to clients which do not support
it.

transcoder/png.go

@@ -3,11 +3,12 @@ package transcoder
 import (
 	"github.com/barnacs/compy/proxy"
 	"image/png"
+	"net/http"
 )
 
 type Png struct{}
 
-func (t *Png) Transcode(w *proxy.ResponseWriter, r *proxy.ResponseReader) error {
+func (t *Png) Transcode(w *proxy.ResponseWriter, r *proxy.ResponseReader, headers http.Header) error {
 	img, err := png.Decode(r)
 	if err != nil {
 		return err