Copies missing headers (namely Content-Type) over to HttpResponse to fix "application/json" being sent out as "text/html".

The response from the auth provider for the token was received as 'text/html' instead on 'application/json' on the client. The result was that the following exception is thrown by the client: "Unexpected response Content-Type text/html".

The cause is that the message's 'Content-Type' header is never copied over from the response message to the HttpResponse being sent out.