title: Handling chained Promise in computed property
body: <p>If I have a then() function on a promise it never resolves properly via a computed property</p>
  
  <p>So this is OK,</p>
  
  <pre><code>  taxRate: computed(function() {
      return this.get('store').findRecord('tax-rate', { current: true });
    })
  </code></pre>
  
  <p>But this is not, (outputs <code>[object Object]</code> - the promise - in the htmlbars template)</p>
  
  <pre><code>  taxRateValue: computed(function() {
      return this.get('store').findRecord('tax-rate', { current: true }).then((taxRate) =&gt; {
        return taxRate.get('taxRateValue');
      })
    })
  </code></pre>
  
  <p>So what is the correct way to handling this without using an observer?</p>