org-agenda: Set default-directory when exporting agenda

* lisp/org-agenda.el (org-agenda-write): Use default-directory of
  export file.  This is necessary for htmlize.el to write the correct
  path to agenda icons.

lisp/org-agenda.el

@@ -3365,6 +3365,7 @@ the agenda to write."
        (save-window-excursion
 	 (let ((bs (copy-sequence (buffer-string)))
 	       (extension (file-name-extension file))
+	       (default-directory (file-name-directory file))
 	       beg content)
 	   (with-temp-buffer
 	     (rename-buffer org-agenda-write-buffer-name t)