title: how to set two class name for Ember.js input helper
body: <p>i'm trying to bind an input element like this:</p>
  
  <pre><code>{{input value=email type="text" placeholder="Enter email" class=emailError:with-error}}
  </code></pre>
  
  <p>it works just fine, as long as I try to assign it only 1 class name ".with-error".
  how can I assign 2 class names, so it will be: ".with-error .second-class"?
  I know how to do it with:</p>
  
  <pre><code>{{bind-attr class=":secondClass emailError:with-error"}}
  </code></pre>
  
  <p>but this doesn't work with input helper.</p>
  
  <p>Thanks!</p>