stg mail: factor out __update_header

stg mail: factor out __update_header

Factor __update_header out of __build_address_headers.

Headers like Reply-To, Mail-Reply-To, and Mail-Followup-To are now
handled in __build_extra_headers.

We make this change because in the future, we do not want to call
__build_address_headers if using git send-email but we will always
want to call __build_extra_headers.

Cc: Karl Wiberg <kha@treskal.com>
Signed-off-by: Alex Chiang <achiang@hp.com>
Signed-off-by: Catalin Marinas <catalin.marinas@gmail.com>