Grow correct buffer size

mtproto/rpc/proxy_request.go

 	bufferLength += bufferLength % 4
 
 	buf := &bytes.Buffer{}
-	buf.Grow(bufferLength)
+	buf.Grow(bufferLength + len(message))
 
 	flags := r.Flags
 	if r.Options.ReadHacks.QuickAck {