Fix message headers when permanently decrypting
* src/mail.cpp (Mail::decryptPermanently_o): Remove content type. * src/mapihelp.cpp (mapi_set_transport_headers), (mapi_set_content_type): New.
While the code allows to set the content type we just remove
it to avoid setting a wrong content type. I hope other
addins handle this properly.
The transport headers are not really used by Outlook after
a Mail is converted into MAPI.
This avoids that GpgOL itself tries to decrypt permanently
decrypted mails again.
- GnuPG-Bug-Id: T4718