fix #2061 add 504 to retriable http status codes

Author: Mpdreamz

src/Elasticsearch.Net/Responses/ElasticsearchResponse.cs

@@ -73,6 +73,7 @@ public class ElasticsearchResponse<T> : IApiCallDetails
 		/// </summary>
 		public bool SuccessOrKnownError =>
 			this.Success || (HttpStatusCode >= 400 && HttpStatusCode < 599
+				&& HttpStatusCode != 504 //Gateway timeout needs to be retried
 				&& HttpStatusCode != 503 //service unavailable needs to be retried
 				&& HttpStatusCode != 502 //bad gateway needs to be retried
 			);

src/Tests/ClientConcepts/ConnectionPooling/Failover/FallingOver.doc.cs

@@ -1,7 +1,4 @@
-using System;
-using System.Threading.Tasks;
-using Elasticsearch.Net;
-using Nest;
+using System.Threading.Tasks;
 using Tests.Framework;
 using static Elasticsearch.Net.AuditEvent;
 
@@ -82,6 +79,30 @@ public async Task Http503FallsOver()
 			);
 		}
 
+		/**[[service-unavailable]]
+		*=== 504 Gateway Timeout
+		*
+		* Will be treated as an error that requires retrying
+		*/
+		[U]
+		public async Task Http504FallsOver()
+		{
+			var audit = new Auditor(() => Framework.Cluster
+				.Nodes(10)
+				.ClientCalls(r => r.FailAlways(504))
+				.ClientCalls(r => r.OnPort(9201).SucceedAlways())
+				.StaticConnectionPool()
+				.Settings(s => s.DisablePing())
+			);
+
+			audit = await audit.TraceCall(
+				new ClientCall {
+					{ BadResponse, 9200 },
+					{ HealthyResponse, 9201 },
+				}
+			);
+		}
+
 		/**
 		* If a call returns a valid http status code other than 502 or 503, the request won't be retried.
 		*