org-publish.el: allow :base-directory to omit the ending slash.

This was spotted by Łukasz Stelmach <lukasz.stelmach@iem.pw.edu.pl>.

lisp/org-publish.el

@@ -472,7 +472,8 @@ matching filenames."
 	(unless (plist-get (cdr prj) :components)
 	  ;; [[info:org:Selecting%20files]] shows how this is supposed to work:
 	  (let* ((r (plist-get (cdr prj) :recursive))
-		 (b (expand-file-name (plist-get (cdr prj) :base-directory)))
+		 (b (expand-file-name (file-name-as-directory
+				       (plist-get (cdr prj) :base-directory))))
 		 (x (or (plist-get (cdr prj) :base-extension) "org"))
 		 (e (plist-get (cdr prj) :exclude))
 		 (i (plist-get (cdr prj) :include))