Add test

Author: pawankartik-elastic

server/src/main/java/org/elasticsearch/action/admin/cluster/stats/ClusterStatsResponse.java

@@ -231,7 +231,7 @@ public XContentBuilder toXContent(XContentBuilder builder, Params params) throws
             builder.startObject();
             builder.field("cluster_uuid", clusterUUID);
             builder.field("mode", mode);
-            builder.field("skip_unavailable", skipUnavailable);
+            // builder.field("skip_unavailable", skipUnavailable);
             builder.field("transport.compress", transportCompress);
             builder.field("status", status);
             builder.field("version", versions);