Pass the dir option on through org-export-as-string

* lisp/org-exp.el (org-export-string): Pass the dir option on through to
  any subsequent export functions.

lisp/org-exp.el

@@ -3047,7 +3047,7 @@ to the value of `temporary-file-directory'."
 	  (eval ;; convert to fmt -- mimicking `org-run-like-in-org-mode'
 	   (list 'let org-local-vars
 		 (list (intern (format "org-export-as-%s" fmt))
-		       nil nil nil ''string t))))
+		       nil nil nil ''string t dir))))
       (delete-file tmp-file))))
 
 ;;;###autoload