title: ng-options trackby in group expression
body: <p>What am I doing wrong here that Angular (1.1.5) is not properly selecting the correct value in this grouped select dropdown?</p>
  
  <p>HTML: <code>&lt;select ng-model="options.metricId" ng-options="answer.id as answer.name group by answer.question for answer in options.answers track by answer.id"&gt;&lt;/select&gt;</code></p>
  
  <p><code>options.metricId</code> is being set properly by the controller.</p>
  
  <p>The <code>options.answers</code> object looks like this: 
  <code>{id: 8, name: "Leads Others", question: "Assessment"}</code></p>
  
  <p>All the options are appearing, but it is not selecting the value where <code>answer.id == options.metricId</code> as I would expect it to. </p>