Support the PreferEmailToUser option on PassUserHeaders

Previously in #401, an option was added to support forwarding the email
address as the username to the upstream service when the PassBasicAuth
option is used.

The PassBasicAuth option is not appropriate for all users, with PassUserHeaders
allowing very similar functionality without specifying a basic auth headers.

The PreferEmailToUser option has been expanded to support the PassUserHeaders
option.