title: Angular 2 observable doesn't 'map' to model
body: <p>As I'm learning Angular 2 I used an observable to fetch some data via an API. Like this:</p>
  
  <pre><code>getPosts() {
          return this.http.get(this._postsUrl)
              .map(res =&gt; &lt;Post[]&gt;res.json())
              .catch(this.handleError);
      }
  </code></pre>
  
  <p>My post model looks is this:</p>
  
  <pre><code>export class Post {
  
  constructor(
      public title: string,
      public content: string,
      public img: string = 'test') {
  }
  </code></pre>
  
  <p>The problem I'm facing is that the map operator doesn't do anything with the Post model. For example, I tried setting a default value for the img value but in the view post.img displays nothing. I even changed Post[] with an other model (Message[]) and the behaviour doesn't change. Can anybody explain this behaviour?</p>