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 and there may be jobs after signing that are changing the mail headers, so we need to copy all headers into the encapsulated part instead of referencing them. BUG: 427091 FIXED-IN: 5.16.1
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