title: Add multiple classes name to post thumbnail in Wordpress
body: <p>this is a wordpress problem. I know I can add a class to a wordpress featured image via this:</p>
  
  <pre><code>&lt;?php the_post_thumbnail('post-thumbnail', ['class' =&gt; 'img-responsive']); ?&gt;
  </code></pre>
  
  <p>However how do I add multiple? I tried using an array but it just displays array in inspecter and I am not sure if it worked. Please help. Thanks.</p>