title: yii2: make checkbox to be checked
body: <p>I am using Yii2 framework and I'd like to generate an html code like this</p>
  
  <pre><code>&lt;input type="checkbox" id="queue-order" name="Queue[order]" value="1" checked&gt;
  </code></pre>
  
  <p>in a view which uses ActiveForm.</p>
  
  <p>I've tried</p>
  
  <pre><code>echo $form-&gt;field($model, 'order')
            -&gt;checkBox(['label' =&gt; ..., 'uncheck' =&gt; null, 'checked' =&gt; true]); 
  </code></pre>
  
  <p>as well as</p>
  
  <pre><code>echo $form-&gt;field($model, 'order')
            -&gt;checkBox(['label' =&gt; ..., 'uncheck' =&gt; null, 'checked' =&gt; 'checked']); 
  </code></pre>
  
  <p>but desired string "checked" does not appear in the generated html code.</p>
  
  <p>Strangely enough, if I substitute "checked" with "selected"</p>
  
  <pre><code>echo $form-&gt;field($model, 'order')
            -&gt;checkBox(['label' =&gt; ..., 'uncheck' =&gt; null, 'selected' =&gt; true]); 
  </code></pre>
  
  <p>then generated html code contains attribute "selected":</p>
  
  <pre><code>&lt;input type="checkbox" id="queue-order" name="Queue[order]" value="1" selected&gt;
  </code></pre>
  
  <p>So, how can I generate html code for a checkbox with attribute "checked"? </p>