response.Headers.HasKeys() returns false even when response.Headers.AllKeys.Length > 0 :scratchhead:

(cherry picked from commit dca1154)

Conflicts:
	src/Elasticsearch.Net/Connection/HttpWebRequestConnection.cs

Author: Mpdreamz

src/Elasticsearch.Net/Connection/HttpWebRequestConnection.cs

@@ -162,7 +162,8 @@ public virtual ElasticsearchResponse<TReturn> Request<TReturn>(RequestData reque
 				builder.StatusCode = (int) response.StatusCode;
 				builder.Stream = response.GetResponseStream();
 
-				if (response.SupportsHeaders && response.Headers.HasKeys() && response.Headers.AllKeys.Contains("Warning"))
+				//response.Headers.HasKeys() can return false even if response.Headers.AllKeys has values.
+				if (response.SupportsHeaders && response.Headers.Count > 0 && 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.