Geary.RFC822.Message: Fix Message-Id header not being set

Merged Michael Gratton requested to merge mjog/758-missing-message-id into mainline

The port to GMime 3 missed setting the Message-Id header on the underlying GMime Message object when constructing a message, from a ComposedEmail, so this was not getting set on outoging email.

Fixes #758 (closed)

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