title: Adding multiple classes with knockout css binding
body: <p>I need to add two classes to an element using the css binding. </p>
  
  <p>For example:</p>
  
  <p><code>&lt;div class="animal" data-bind="css: mammal, dog"&gt;&lt;/div&gt;</code></p>
  
  <p>However this only adds the first class, "mammal".</p>
  
  <p>Please could you tell me what I'm doing wrong. Thanks </p>