Fix handling of MIMEData without Headers
* src/mimedataprovider.cpp (MimeDataProvider): Add ctor variants with no_headers field. * src/mimedataprovider.h: Update Accordingly. * src/parsecontroller.cpp (expect_no_headers): Expect no headers except for some messages. (decrypt_template): New to be used for status messages.
This covers the problem that GpgOL only sees crypto data as an
attachment and not the original outer mime structure.