Add option to set withCredentials to true without setting username for auth

Firefox sends the username for auth even though a password was not supplied
which causes Firefox to abort before sending a preflight request for Cross-Origin
requests.

See http://stackoverflow.com/questions/11255173/firefox-wont-send-cross-origin-resource-sharing-pre-flight
Deprecate setting xhr.withCredentials flag from HTTP auth
Add unit tests for xhr.withCredentials behavior
Update request documentation
Merge pull request #2486 from vimeo/master

Add option to trigger xhr.withCredentials without http auth