[15.0] mail_debrand sends mail in text/plain in e-mail marketing app

[bjornbillen]

Module

mail_debrand

Describe the bug

In the e-mail marketing application, an e-mail is sent with wrong content-type.
The recipient gets the e-mail without HTML markup.

To Reproduce

15.0 EE

Steps to reproduce the behavior:

1. Create a e-mail marketing campaign
2. Select a default mailing template
3. Send a test e-mail
4. Receive and inspect the e-mail headers and look for "text/plain"

Expected behavior
The e-mail should be rendered in HTML.

[github-actions]

There hasn't been any activity on this issue in the past 6 months, so it has been marked as stale and it will be closed automatically if no further activity occurs in the next 30 days.
If you want this issue to never become stale, please ask a PSC member to apply the "no stale" label.

[pedrobaeza]

Should be fixed in #939