Bad requests cause a NullReferenceException in ElasticClient

[gmarz]

After some recent changes (possibly #633), bad requests (ElasticsearchResponse.Success == false) are throwing a NullReferenceException in ElasticClient.ResultsSelector<D, Q, R> if no RequestConfiguration is setup on the descriptor, because ElasticsearchResponse.Response is null and allowStatusCode evaluates to true.

If it's correct that the Response property should be null for invalid requests, then I think allowStatusCode should default to false when there is no request configuration.