Ah okay, if you leave the password field blank but don't have one set,
our simple `update` method gets annoyed that you left it blank.
In this change, we simplify the model API by just overriding
`update_with_password` with our own special behavior for the
no-password case.
I'm getting ready to add handling for "what if you don't *have* a
current password*??", so it seems like the right way to do that is to
just eject the controller and start customizing!
