Fix bug with case-sensitive header comparisions (#5940)

* Fix bug with case-sensitive header comparisons
* OrdinalIgnoreCase vs InvariantCultureIgnoreCase

Author: jadanah

src/Elasticsearch.Net/Connection/HttpWebRequestConnection.cs

@@ -72,12 +72,13 @@ public virtual TResponse Request<TResponse>(RequestData requestData)
 				HandleResponse(httpWebResponse, out statusCode, out responseStream, out mimeType);
 
 				//response.Headers.HasKeys() can return false even if response.Headers.AllKeys has values.
-				if (httpWebResponse.SupportsHeaders && httpWebResponse.Headers.Count > 0 && httpWebResponse.Headers.AllKeys.Contains("Warning"))
+				if (httpWebResponse.SupportsHeaders && httpWebResponse.Headers.Count > 0 
+					&& httpWebResponse.Headers.AllKeys.Contains("Warning", StringComparer.OrdinalIgnoreCase))
 					warnings = httpWebResponse.Headers.GetValues("Warning");
 
 				//response.Headers.HasKeys() can return false even if response.Headers.AllKeys has values.
 				if (httpWebResponse.SupportsHeaders && httpWebResponse.Headers.Count > 0
-					&& httpWebResponse.Headers.AllKeys.Contains("X-elastic-product"))
+					&& httpWebResponse.Headers.AllKeys.Contains("X-elastic-product", StringComparer.OrdinalIgnoreCase))
 					productNames = httpWebResponse.Headers.GetValues("X-elastic-product");
 			}
 			catch (WebException e)
@@ -152,12 +153,13 @@ CancellationToken cancellationToken
 
 					var httpWebResponse = (HttpWebResponse)await apmGetResponseTask.ConfigureAwait(false);
 					HandleResponse(httpWebResponse, out statusCode, out responseStream, out mimeType);
-					if (httpWebResponse.SupportsHeaders && httpWebResponse.Headers.HasKeys() && httpWebResponse.Headers.AllKeys.Contains("Warning"))
+					if (httpWebResponse.SupportsHeaders && httpWebResponse.Headers.HasKeys() 
+						&& httpWebResponse.Headers.AllKeys.Contains("Warning", StringComparer.OrdinalIgnoreCase))
 						warnings = httpWebResponse.Headers.GetValues("Warning");
 
 					//response.Headers.HasKeys() can return false even if response.Headers.AllKeys has values.
 					if (httpWebResponse.SupportsHeaders && httpWebResponse.Headers.Count > 0
-						&& httpWebResponse.Headers.AllKeys.Contains("X-elastic-product"))
+						&& httpWebResponse.Headers.AllKeys.Contains("X-elastic-product", StringComparer.OrdinalIgnoreCase))
 						productNames = httpWebResponse.Headers.GetValues("X-elastic-product");
 				}
 			}