Avoid a deprecated error when the attribute name is numeric and Direc…

…tLex is used (Fix ezyang#393)

library/HTMLPurifier/Token/Tag.php

@@ -44,7 +44,7 @@ public function __construct($name, $attr = array(), $line = null, $col = null, $
         $this->name = ctype_lower($name) ? $name : strtolower($name);
         foreach ($attr as $key => $value) {
             // normalization only necessary when key is not lowercase
-            if (!ctype_lower($key)) {
+            if (!ctype_lower((string)$key)) {
                 $new_key = strtolower($key);
                 if (!isset($attr[$new_key])) {
                     $attr[$new_key] = $attr[$key];