Setting variable for local namespaces to protected in writer.

The namespaces for mathml, html, and svg are set to protected
so they cannot be altered. These are part of the spec.

Author: mattfarina

src/HTML5/Serializer/Traverser.php

@@ -16,7 +16,7 @@ class Traverser
     /**
      * Namespaces that should be treated as "local" to HTML5.
      */
-    static $local_ns = array(
+    protected static $local_ns = array(
         'http://www.w3.org/1999/xhtml' => 'html',
         'http://www.w3.org/1998/Math/MathML' => 'math',
         'http://www.w3.org/2000/svg' => 'svg'