The issue occurs when calling URI such as /layout/set/<layout>/<uri>.
The Content-Type header set in kernel/layout/set.php is not 'forwarded' to the eZ Publish 5 response object.
Step to reproduce :
- Customize Content-Type header in kernel/layout/set.php
- Call the layout set, for example : http://mysite.com/layout/set/print/content/view/full/118
- Look at headers : the customized header won't be there.