title: Wordpress widget: Checkbox not saving state
body: <p>I have a Wordpress widget with checkboxs. I am trying to set the default state to "checked" for one of the checkboxes by setting a variable in my defaults array to true. If I uncheck the box and try to save the widget the checkbox reverts back to being checked, so basically I can't uncheck the box. </p>
  
  <p>I have other checkboxs with the same configuration and they are working correctly. Does anyone see anything wrong with my code below?</p>
  
  <pre><code>$defaults = array(
      'widget' =&gt; 'cat',
      'type' =&gt; 'list',
      'selected' =&gt; 0,
      'use_products' =&gt; 'true',
      'title' =&gt; '',
      'sub_title' =&gt; '',
      'operator' =&gt; 'IN',
      'print_before_widget' =&gt; 'true',
      'print_after_widget' =&gt; 'true',
      'hide_empty' =&gt; 'true',
      'hide_on_cats' =&gt; 'false',
      'custom_class' =&gt; ''
  );
  
  $instance = parse_args( $instance, $defaults );
  extract( $instance );
  
  &lt;p&gt;
  &lt;input class="checkbox"
          &lt;?php if ( isset( $use_products ) &amp;&amp; $use_products=='true' ) { echo 'checked="checked"'; } ?&gt;
          id = "&lt;?php echo $this-&gt;get_field_id( 'use_products' ); ?&gt;"
          name = "&lt;?php echo $this-&gt;get_field_name( 'use_products' ); ?&gt;"
          value = "true"
          type = "checkbox"
      /&gt;
      &lt;label for="&lt;?php echo $this-&gt;get_field_id( 'use_products' ); ?&gt;" &gt;&lt;?php _e( 'Show Product Categories', textdomain ); ?&gt;&lt;/label&gt;
  &lt;/p&gt;
  </code></pre>