org-gnus.el: Add the :date property.

* org-gnus.el (org-gnus-store-link): add the :date property to
gnus links, allowing the use of %:date in capture templates.

lisp/org-gnus.el

@@ -167,7 +167,7 @@ If `org-store-link' was called with a prefix arg the meaning of
 	(setq to (or to (gnus-fetch-original-field "To"))
 	      newsgroups (gnus-fetch-original-field "Newsgroups")
 	      x-no-archive (gnus-fetch-original-field "x-no-archive")))
-      (org-store-link-props :type "gnus" :from from :subject subject
+      (org-store-link-props :type "gnus" :from from :subject subject :date date
 			    :message-id message-id :group group :to to)
       (setq desc (org-email-link-description)
 	    link (org-gnus-article-link