title: I am having issues handling a couple deprecation warnings in v2.1.0
body: <p>I can not figure out how to handle these deprecation warnings:
  <a href="https://i.stack.imgur.com/crLN3.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/crLN3.png" alt=""></a></p>
  
  <p><a href="https://i.stack.imgur.com/VBleq.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/VBleq.png" alt=""></a></p>
  
  <p>Here is my serializer:</p>
  
  <pre><code>import DS from 'ember-data';
  
  export default DS.RESTSerializer.extend({
    primaryKey: '_id'
  });
  </code></pre>