title: add dynamic class with css binding
body: <p>I want to add a class behind <code>payment-method</code> by function with the knockout css binding (in Magento 2.1).
  So this is the current code:</p>
  
  <pre><code>&lt;div class="payment-method" data-bind="css: {'_active': (getCode() == isChecked())}"&gt;
      &lt;div class="payment-method-title field choice"&gt;
          &lt;input type="radio"
                 name="payment[method]"
                 class="radio"
                 data-bind="attr: {'id': getCode()}, value: getCode(), checked: isChecked, click: selectPaymentMethod, visible: isRadioButtonVisible()"/&gt;
  </code></pre>
  
  <p>The class is returned by <code>getCode()</code> which works above with the id and value.
  So I thought I could do just:</p>
  
  <pre><code>&lt;div class="payment-method" data-bind="css: {'_active': (getCode() == isChecked()), getCode()}"&gt;
  </code></pre>
  
  <p>But then it fails with:</p>
  
  <blockquote>
    <p>knockout.js:2624 Uncaught SyntaxError: Unable to parse bindings.
    Bindings value: css: {'_active': (getCode() == isChecked()), getCode() }
    Message: Unexpected token }</p>
  </blockquote>
  
  <pre><code>&lt;div class="payment-method" data-bind="css: getCode()"&gt;
  </code></pre>
  
  <p>This works.</p>
  
  <pre><code>&lt;div class="payment-method" data-bind="css: {getCode()}"&gt;
  </code></pre>
  
  <p>This doesn't.</p>
  
  <pre><code>&lt;div class="payment-method" data-bind="css: {'_active': (getCode() == isChecked())}, attr: {'class': getCode()}"&gt;
  </code></pre>
  
  <p>This works too but will overwrite the <code>payment-method</code> class and the <code>_active</code> class isn't set either initally anymore.</p>
  
  <p>How do I set that dynamic class?</p>