Simplify / fix S/MIME data collection.

* src/mimedataprovider.cpp: Copy everything if expect
no headers is true.
* src/parsecontroller.cpp (expect_no_headers): Only expect
headers for multipart signed.

Besides multipart signed outlook only puts the crypto part in
the attachment that we treat as MOSS attachment. So we don't
see any headers and parsing this as text can lead to problems
for smime where the content is binary.