Fix middleware when used with prettyfier and gzip encoding

If while decompressing gzip payload decompressed value was less then
compressed, it did not changed buffer length, and middleware received
bytes left from gzip response.

middleware.go

@@ -71,6 +71,7 @@ func (m *Middleware) copy(to io.Writer, from io.Reader) {
 
 			if Settings.prettifyHTTP {
 				payload = prettifyHTTP(payload)
+                nr = len(payload)
 			}
 
 			hex.Encode(dst, payload)