Returns empty uuid for "on-behalf-of" user

When request is made with "on-behalf-of" header creds.Uuid() gives empty string for "local" user.

Looking at the "VerifyOnBehalf()" call it will just populate the User and Domain field.
https://github.com/couchbase/cbauth/blob/2821aa6c2ce9332d877a587b7ab2a0ac4fdb2969/cbauthimpl/impl.go#L618