Fix write-document to work for feeds.

src/documents.lisp

 (defun write-document (document &optional theme-fn &rest render-args)
   "Write the given DOCUMENT to disk as HTML. If THEME-FN is present,
 use it as the template passing any RENDER-ARGS."
-  (let ((html (if render-args
+  (let ((html (if (or theme-fn render-args)
                   (apply #'render-page document theme-fn render-args)
                   (render-page document nil))))
     (write-file (page-path obj) html)))