Fix postpone/resume to not remove a Mail-Followup-To header (closes #3070)

Fix postpone/resume to not remove a Mail-Followup-To header  (closes #3070)

This solution changes mutt_prepare_template() to check whether the
message_id field is NULL to decide whether to wipe the message-id and
mail-followup-to headers when instantiating the message.

If we are resending a message, we don't want the previous message-id
and mail-followup-to headers.  If we are resuming a postponed message,
however, we want to keep the mail-followup-to header if any was set
before the postpone.