Fix gnus-group-get-new-news-this-group on group with closed server

* lisp/gnus/nnimap.el (nnimap-change-group): Lookup server
method and pass to nnimap-open-server.

lisp/gnus/nnimap.el

@@ -1831,7 +1831,9 @@ Return the server's response to the SELECT or EXAMINE command."
   (let ((open-result t))
     (when (and server
 	       (not (nnimap-server-opened server)))
-      (setq open-result (nnimap-open-server server nil no-reconnect)))
+      (let ((method (gnus-server-to-method server)))
+	(setq open-result (nnimap-open-server (nth 1 method) (nthcdr 2 method)
+					      no-reconnect))))
     (cond
      ((not open-result)
       nil)