Fix[messagecomposer]: Do copy all mail headers instad of reference them.

When dealing with encryption and protected headers it is enough to reference the headers in the messagepart, as the result is encrypted directly. This is different for sign only as there may be jobs after signing that are changing the outside mail headers, so we need to copy all headers into the encapsulated part instead of referencing them to make sure, that the signature is not broken.

BUG: 427091 FIXED-IN: 5.16.1