Set a default message sender in no sender set in DebugMailer

Use-case: I construct Messages in my code without a sender, because I want to
use the ``default_sender`` as the sender. Works as a charm with ``Mailer``.

However, when I use ``DebugMailer`` with these Messages, I get a message a
validation error, because the sender is not set. The ``Mailer`` class takes
care of setting the sender if default sender is set, but the ``DebugMailer``
does not do that. This patch then sets a dummy sender, if no sender is set in
Message, to allow using ``DebugMailer`` with sender-less Messages.