This is more like the behavior of the old AuthBrute mixin, where a
scanner module was expected to return :next_user in the block given to
each_user_pass when it successfully authenticated.
The advantage is a reduced number of attempts that are very unlikely to
be successful since we already know the password. However, note that
since we don't compare realms, this will cause a false negative in the
rare case where the same username exists with different realms on the
same service.
MSP-10686
made the one true enumerator of credentials
for the login_scanner.
also covered the wierd http case where it can have a realm key
but no default realm.
James Lee