[BUGFIX] Add Content-Type to test mail plain part

Resolves: #85536
Releases: master, 8.7
Change-Id: Ie1f5c97af1a315eb5b8d7bf4bc7a31872362750d
Reviewed-on: https://review.typo3.org/57536


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Markus Klein <markus.klein@typo3.org>
Tested-by: Markus Klein <markus.klein@typo3.org>
Reviewed-by: Josef Glatz <josef.glatz@typo3.org>
Tested-by: Josef Glatz <josef.glatz@typo3.org>
Reviewed-by: Mathias Brodala <mbrodala@pagemachine.de>
Tested-by: Mathias Brodala <mbrodala@pagemachine.de>