Inbound Mail: Problems with diazo themes.

When using a diazo theme the inbound-mail functionality does not work correctly. Even thoug the object is correctly created, the [mta2plone](https://github.com/4teamwork/ftw.mail/blob/master/ftw/mail/mta2plone.py) script raise and `ValueError` (Unknow error).

This because the response content-type is not correctly set to `text/plain` in the [inbound view](https://github.com/4teamwork/ftw.mail/blob/master/ftw/mail/inbound.py#L40)