Suppressing WWW-Authenticate using X-Requested-With: XMLHttpRequest: source?

This tutorial states that appending X-Requested-With: XMLHttpRequest to the HTTP request prevents the (spring) server from responding with a WWW-Authenticate.

I am wondering:

1. Is this behaviour documented? Can anyone point me to a source of reference?
2. Is it a standard or a convention in web servers? is it spring-specific?

Thanks!