title: how to add a class into the existing classes based on a bool condition in angular 2
body: <p>How do you place the "active" class in an element based on a typeScript boolean condition?</p>
  
  <p>My use case:</p>
  
  <p>Instead of just the <code>class="collapsible-header "</code> like this</p>
  
  <pre><code>&lt;div class="collapsible-header " &gt;
  My Title
  &lt;/div&gt;                                          
  </code></pre>
  
  <p>I would like this to be written:</p>
  
  <pre><code>&lt;div class="collapsible-header active" &gt;
  My Title
  &lt;/div&gt;                                          
  </code></pre>
  
  <p>As you see there is also the "active" class in there. 
  But this should appear only and only if myBool is true</p>
  
  <p>How do we make this happen?</p>