title: Ember FixtureAdapter error: cannot assign new id
body: <p>In my ember integration tests I'm getting this error:</p>
  
  <p>Assertion failed: An adapter cannot assign a new id to a record that already has an id.  had id: fixture-0 and you tried to update it with null. This likely happened because your server returned data in response to a find or update that had a different id than the one you sent. </p>