warnings caused by incorrect assumption about $default_fields being an array

Seeing a couple of PHP warnings because of $default_fields being treated as an array when it looks like that is not always the case:

Warning: implode(): Invalid arguments passed in EntityQueueTaxonomyEntityQueueHandler->subqueueForm() (line 99 of /path/to/drupal-7.x/sites/all/modules/contrib/entityqueue_taxonomy/plugins/entityqueue/handler/EntityQueueTaxonomyEntityQueueHandler.class.php).

Warning: Invalid argument supplied for foreach() in form_type_checkboxes_value() (line 2392 of /path/to/drupal-7.x/includes/form.inc).

Patch on the way.