proxy header-only responses properly

fixes redirects

transcoder/identity.go

@@ -2,12 +2,10 @@ package transcoder
 
 import (
 	"github.com/barnacs/compy/proxy"
-	"io"
 )
 
 type Identity struct{}
 
 func (i *Identity) Transcode(w *proxy.ResponseWriter, r *proxy.ResponseReader) error {
-	_, err := io.Copy(w, r)
-	return err
+	return w.ReadFrom(r)
 }