Fix Not included in the search criteria "type = lookup" in $filterArgs

The following cases, 'blog_id' is not included in the search criteria.

ex)
```PHP
ArticlesController->$presetVars = true;

Article->$filterArgs = array(
    'blog_id' => array('type' => 'lookup', 'formField' => 'blog_input', 'modelField' => 'title', 'model' => 'Blog')
);
```