bugfix: flow_stats test needed to set buffer_id

Thanks to Thu Quoc <tquoc@us.ibm.com> for pointing this out.
diff --git a/tests/caps.py b/tests/caps.py
index 14d616f..d0bb2d0 100644
--- a/tests/caps.py
+++ b/tests/caps.py
@@ -75,6 +75,7 @@
         match.wildcards |= ofp.OFPFW_DL_SRC
 
     request.match = match
+    request.buffer_id = 0xffffffff      # set to NONE
     caps_logger.info(request.show())
 
     tstats = message.table_stats_request()