Authorization Header is getting stripped from request somewhere between begin_request and DelegatingHandler.SendAsync. Im trying to debug why this is happening.
I have a Delegating Handler in my WebApi designed to validate the Json WebToken. Unfortunately when SendAsync(HttpRequestMessage request, CancellationToken cancellationToken) fires the Authorization property of the request is null.
However in Application Begin Request the Authorization Header appears to be received from the client.
Not a complete answer but http://jameschambers.com/2013/11/working-with-iauthenticationfilter-in-the-mvc-5-framework/
Answered a portion of what occurs before the Delegating Handler that calls the third party. By creating a ActionFilterAttribute, IAuthenticationFilter the Authentication can be intercepted and handled in whatever custom way is desired.