Added missing call to MimeMessage.saeChanges

According to JavaDoc
http://docs.oracle.com/javaee/6/api/javax/mail/Transport.html#sendMessage:

"Unlike the static send method, the sendMessage method does not call the
saveChanges method on the message; the caller should do so. "

The call to saveChanges was missing here. The user of Aspirin cannot
know that this call is necessary and therefore it is necessary to make
the call here.