when setting null value ttl, reset it

modules/elasticsearch/src/main/java/org/elasticsearch/action/index/IndexRequest.java

@@ -288,15 +288,19 @@ public class IndexRequest extends ShardReplicationOperationRequest {
         return this.timestamp;
     }
 
-    // Sets the relative ttl value. It musts be > 0 as it makes little sense otherwise.
+    /**
+     * Sets the relative ttl value. It musts be > 0 as it makes little sense otherwise. Setting it
+     * to <tt>null</tt> will reset to have no ttl.
+     */
     public IndexRequest ttl(Long ttl) throws ElasticSearchGenerationException {
         if (ttl == null) {
+            this.ttl = -1;
             return this;
         }
         if (ttl <= 0) {
             throw new ElasticSearchIllegalArgumentException("TTL value must be > 0. Illegal value provided [" + ttl + "]");
         }
-        this.ttl  = ttl;
+        this.ttl = ttl;
         return this;
     }