MB-10721 ignore basic auth headers for ui issued xhrs

UI doesn't use basic auth, but the browser can still attach
basic auth header which will confuse token based auth

Change-Id: Iba2c256c51d8102593f0d882ae164bfa77a605d6
Reviewed-on: http://review.couchbase.org/36120
Tested-by: Artem Stemkovski <artem@couchbase.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>