title: collection check boxes in view select
body: <p>in the view</p>
  
  <pre><code>= form.collection_check_boxes :category_ids, Item::Category.all, :id, :name, checked: "#{@search.try(:category_ids)}"
  </code></pre>
  
  <p>returns</p>
  
  <pre><code>"[\"3\", \"\"]"
  </code></pre>
  
  <p>but it should produce</p>
  
  <pre><code>["3", ""]
  </code></pre>
  
  <p>Why does it happen?</p>
  
  <pre><code>@search.try(:category_ids) is ["3", ""]
  </code></pre>