Updated the issue title to reflect the current state of the proposal - adding a new Handler class for use when you want to send the auth details unconditionally, rather than requiring that the server send a 401 response before resubmitting the request with authentication attached.

The use of a separate class addresses the concerns around sending credentials unconditionally, and the restriction to 3.5+ matches the general conclusion that it's a new feature.

I'll commit this shortly (just making sure my local build environment is properly configured after upgrading to the F21 beta release).