title: Activeadmin save value from checkbox as boolean
body: <p>I want to save value from my checkbox's form as boolean, but i have got an array</p>
  
  <p>Chunk of my log <code>"important"=&gt;[""]</code></p>
  
  <p>or if i checked it</p>
  
  <p><code>"important"=&gt;["", "true"]</code></p>
  
  <p>my table </p>
  
  <pre><code>t.boolean :important, :default =&gt; false
  </code></pre>
  
  <p>my form </p>
  
  <pre><code>f.input :important, :as =&gt; :check_boxes, collection: [true]
  </code></pre>
  
  <p>How to fix it?</p>
  
  <p>i think problem in my form's input</p>