Use node toString if id is undefined

Author: Hypercubed

lib/search.js

@@ -49,7 +49,9 @@ function search(root, expression, maxDepth) {
      */
 
     visit(root, HEADING, function (child, index, parent) {
-        var value = (child.data && child.data.hProperties) ? child.data.hProperties.id : toString(child);
+        var value = (child.data && child.data.hProperties && child.data.hProperties.id) ?
+            child.data.hProperties.id :
+            toString(child);
         var id = slugs.slug(value);
 
         if (parent !== root) {