Make sure we provide the right return value for callbacks also.

Also, don't mutate `name` to "update" from "upsert"; the function closes over it
so all subsequent calls will start with name "update".
Update minimongo tests to new api.

All tests pass now, but we've got the unfortunate situation that the
LocalCollection return values don't match the Meteor.Collection return values,
which is problematic because the minimongo tests use LocalCollection
directly. Hmm.