LP#1620803 Add missing workstation passthru to AuthProxy

LP#1620803 Add missing workstation passthru to AuthProxy

The AuthProxy.pm login function was not passing through a possible
'workstation' parameter to auth_internal's 'session.create' method.
Without this, all login's end up with a workstation set to the user's
home_ou, which can affect things including opt-in and transit
behaviors.

While not needed for the initial 'user.validate', it seems cleaner
and simpler to just bring it over when constructing the initial
$args hash.

Signed-off-by: Dan Wells <dbw2@calvin.edu>