Conflict with other plugins accessing $data_index object as a SimpleXML object

[lokothodida]

Example

I18N Special Pages has a getFields method which expects the $data_index object to be a SimpleXML object:

    // i18n_specialpages/specialpages.class.php
    // at roughly line 275 ($data is $data_index)
      foreach ($data->children() as $child) {
        if (!in_array($child->getName(), $stdfields) && (string) $child) {
          $fields[$child->getName()] = (string) $child;
        }
      }

The current FrontRouter code makes $data_index a plain object by merging two (cast) arrays, so the merge should instead just bolt these properties on, instead of doing a full merge.