title: ngClass is not working like it should
body: <p>I'm playing around with ngClass, and I  do not know what I am doing wrong. It is showing weird behavior. It seems like when I assign a function to ngClass it does not work, but if I assign it an object literal it works. How can I get ngClass to work with my Component's function?</p>
  
  <p><strong>ProposalForm.ts</strong></p>
  
  <pre><code>import {Component} from 'angular2/core';
  import {NgClass}    from 'angular2/common';
  @Component({
    selector: 'proposal-form',
    template: `
     &lt;div [ngClass]="setClasses()"&gt;
      ghtfhg
    &lt;/div&gt;
    &lt;div [ngClass]="{active:true, disabled:true}"&gt;
      1111111
    &lt;/div&gt;`,
    directives:[NgClass]
  })
  export class ProposalForm {
  
    setClasses() {
     return {active:true, disabled:true};
    }
  }
  </code></pre>
  
  <p>Looking at Chrome's Inspector this is the HTML</p>
  
  <pre><code>&lt;proposal-form _ngcontent-xxb-2=""&gt;
  &lt;div class=""&gt;
    ghtfhg
  &lt;/div&gt;
  &lt;div class="active disabled"&gt;
    1111111
  &lt;/div&gt;&lt;/proposal-form&gt;
  </code></pre>