Read correct parameter for causal_reads

The configuration used the wrong parameter name. The test also did not
explicitly enable tracking of the last_gtid variable which caused it to
fail if it wasn't already on.

server/modules/routing/readwritesplit/rwsplitsession.cc

@@ -363,9 +363,7 @@ GWBUF* RWSplitSession::handle_causal_read_reply(GWBUF *writebuf, SRWBackend& bac
     {
         if (GWBUF_IS_REPLY_OK(writebuf) && backend == m_current_master)
         {
-            /** Save gtid position */
-            char *tmp = gwbuf_get_property(writebuf, MXS_LAST_GTID);
-            if (tmp)
+            if (char *tmp = gwbuf_get_property(writebuf, MXS_LAST_GTID))
             {
                 m_gtid_pos = std::string(tmp);
             }