Added DigestAjax.WWW_AUTHENTICATE variable to hold "WWW-Authenticate" header name. Changing this variable will change the header digest-ajax retrieves (e.g. when using a different header name to circumvent built-in browser authentication prompts).

Additionally, the returning promised is wrapped in the unauthenticated request's XHR object and returned. This is useful if a function like abort needs to be called, but it won't stop the second authenticated request.