Autoload url-insert-buffer-contents

* lisp/url/url-handlers.el: Add autoload cookie so that
`package-list-packages' doesn't bug out (bug#21927) (tiny change)

lisp/url/url-handlers.el

@@ -309,6 +309,7 @@ They count bytes from the beginning of the body."
 
 (defvar url-http-codes)
 
+;;;###autoload
 (defun url-insert-buffer-contents (buffer url &optional visit beg end replace)
   "Insert the contents of BUFFER into current buffer.
 This is like `url-insert', but also decodes the current buffer as