Null reference exception thrown by request error #138
Description
If the HTTP request to the API fails, a NullReferenceException is thrown by CyberSource.Client.ApiClient.CallApiAsync.
The easiest way to reproduce this is to use a non-existent URL. However, we have experienced this in production due to connectivity issues.
This is caused by the code attempting to enumerate the response headers collection without checking for the headers collection being null.