title: Can't get key - angularfire2
body: <p>Hello I am using angularfire2 + firebase into my ionic3 app, I am tryng to get key (for example: C0u7DXjuejQjfCBHBnwL9xK3TCi1) but can't</p>
  
  <p>not working:</p>
  
  <pre><code>&lt;li *ngFor="let client of clients | async"&gt;
     {{client.key}}
  &lt;/li&gt;
  </code></pre>
  
  <p>I tired also <code>client.$key</code> but not working..</p>
  
  <p>but I am getting values good. for example:</p>
  
  <pre><code>&lt;li *ngFor="let client of clients | async"&gt;
     {{client.name}}
  &lt;/li&gt;
  </code></pre>