mu4e-comnpose: avoid jumping to headers when saving

Wrap the before-save-hook function in a save-excursion.

(HT: Joost Kremers)

mu4e/mu4e-compose.el

@@ -303,10 +303,11 @@ Message-ID."
   (add-hook 'before-save-hook
     (lambda()
       ;; replace the date
+      (save-excursion
         (message-remove-header "Date")
         (message-generate-headers '(Date Message-ID))
         (save-match-data
-        (mu4e~draft-remove-mail-header-separator))) nil t)
+          (mu4e~draft-remove-mail-header-separator)))) nil t)
   (add-hook 'after-save-hook
     (lambda ()
       (save-match-data