mu4e-view: assign gnus-article-buffer in mu4e-view-mode

Some external programs (such as bbdb) seems to depend on that.

mu4e/mu4e-view.el

@@ -1050,6 +1050,9 @@ This is useful for advising some Gnus-functionality that does not work in mu4e."
 (define-derived-mode mu4e-view-mode gnus-article-mode "mu4e:view"
   "Major mode for viewing an e-mail message in mu4e.
 Based on Gnus' article-mode."
+  ;; some external tools (bbdb) depend on this
+  (setq gnus-article-buffer (current-buffer))
+
   ;; ;; turn off gnus modeline changes and menu items
   (advice-add 'gnus-set-mode-line :around #'mu4e--view-nop)
   (advice-add 'gnus-button-reply :around #'mu4e--view-button-reply)