Previous commit only fixed Request not RequestAsync's way of handling response headers

Author: Mpdreamz

src/Elasticsearch.Net/Connection/HttpConnection-CoreFx.cs

@@ -70,7 +70,9 @@ public virtual ElasticsearchResponse<TReturn> Request<TReturn>(RequestData reque
 				var response = client.SendAsync(requestMessage).GetAwaiter().GetResult();
 				requestData.MadeItToResponse = true;
 				builder.StatusCode = (int)response.StatusCode;
-				builder.DeprecationWarnings = response.Headers.GetValues("Warning");
+				IEnumerable<string> warnings;
+				if (response.Headers.TryGetValues("Warning", out warnings))
+					builder.DeprecationWarnings = warnings;
 
 				if (response.Content != null)
 					builder.Stream = response.Content.ReadAsStreamAsync().GetAwaiter().GetResult();

src/Elasticsearch.Net/Connection/HttpConnection.cs

@@ -204,7 +204,8 @@ public virtual async Task<ElasticsearchResponse<TReturn>> RequestAsync<TReturn>(
 				var response = (HttpWebResponse)(await apmGetResponseTask.ConfigureAwait(false));
 				builder.StatusCode = (int)response.StatusCode;
 				builder.Stream = response.GetResponseStream();
-				builder.DeprecationWarnings = response.Headers.GetValues("Warning");
+				if (response.SupportsHeaders && response.Headers.HasKeys() && response.Headers.AllKeys.Contains("Warning"))
+					builder.DeprecationWarnings = response.Headers.GetValues("Warning");
 				// https://github.com/elastic/elasticsearch-net/issues/2311
 				// if stream is null call dispose on response instead.
 				if (builder.Stream == null || builder.Stream == Stream.Null) response.Dispose();