apply fix suggested by @ibauersachs

Author: pjfanning

hadoop-common-project/hadoop-registry/src/main/java/org/apache/hadoop/registry/server/dns/RegistryDNS.java

@@ -1682,7 +1682,7 @@ public void exec(Zone zone, Record record) throws IOException {
                   DNSSEC.sign(rRset, dnskeyRecord, privateKey,
                       inception, expiration);
               LOG.info("Adding {}", rrsigRecord);
-              rRset.addRR(rrsigRecord);
+              zone.addRecord(rrsigRecord);
 
               //addDSRecord(zone, record.getName(), record.getDClass(),
               //  record.getTTL(), inception, expiration);