fixed a bug with getCategories(), getCollections() and getCopies()

When calling getCategories(), getCollections() or getCopies() without an id, the API always returns "404". With this patch, these calls work as expected and return lists of all categories, collections or copies.

$tiv = new Thingiverse($access_token);
$res = $tiv->getCategories();
$res = $tiv->getCollections();
$res = $tiv->getCopies();