[BUGFIX] Fix assignment in class.dmailer.php

Additionally a unit test has been created for the method being
changed. This is the first unit test of the direct_mail extension
but further tests should get added for every change AFAP.

Change-Id: I1acdc6994ef2efb588317dc2e9b9973e7ede2de5
Resolves: #53173
Reviewed-on: https://review.typo3.org/26377
Reviewed-by: Bernhard Kraft
Tested-by: Bernhard Kraft
Reviewed-by: Stefan Neufeind