DSL:MailSendingMessageHandler

How about two overloaded version of one method?
```
MailSendingMessageHandlerSpec credentials(String user) {...}

MailSendingMessageHandlerSpec credentials(String user, String password) {...}
```
?
Or we really can use `password` without `user`?
BTW, should we add `Assert`s for them in the `MailSendingMessageHandlerSpec` ?