Fix to accept transaction_id to all message structs

lib/openflow/multipart/group_desc/request.ex

@@ -11,8 +11,8 @@ defmodule Openflow.Multipart.GroupDesc.Request do
 
   def ofp_type, do: 18
 
-  def new do
-    %Request{}
+  def new(xid \\ 0) do
+    %Request{xid: xid}
   end
 
   def read("") do