title: ngOptions setting value attribute of option to unexpected values
body: <p>I have some doubts with using ngoptions. I am not able to set value attribute for the option items.Here is <a href="http://plnkr.co/edit/BUjOCGSl5sBM8VWVef7F?p=preview" rel="nofollow">example plunker</a></p>
  
  <pre><code>  $scope.ListOfValues=[{optiontext:'Active',optionvalue:'opt1'},
       {optiontext:'inactive',optionvalue:'opt2'},
       {optiontext:'terminated',optionvalue:'opt3'}];
  </code></pre>
  
  <p>And my html code is </p>
  
  <pre><code> &lt;select id="emptype" ng-model="empstatus" ng-options="emp.optionvalue as emp.optiontext  for emp in ListOfValues"&gt;
  &lt;/select&gt;
  </code></pre>
  
  <p>The generated html is as shown below.</p>
  
  <pre><code>&lt;select id="emptype" ng-model="empstatus" ng-options="emp.optionvalue as emp.optiontext  for emp in ListOfValues" class="ng-valid ng-dirty ng-valid-parse ng-touched"&gt;
    &lt;option value="string:opt1" label="Active"&gt;Active&lt;/option&gt;
    &lt;option value="string:opt2" label="inactive"&gt;inactive&lt;/option&gt;
    &lt;option value="string:opt3" label="terminated"&gt;terminated&lt;/option&gt;
  &lt;/select&gt;
  </code></pre>
  
  <p>I was expecting it to be as shown below</p>
  
  <pre><code>&lt;select id="emptype" ng-model="empstatus" ng-options="emp.optionvalue as emp.optiontext  for emp in ListOfValues" class="ng-valid ng-dirty ng-valid-parse ng-touched"&gt;
        &lt;option value="opt1" label="Active"&gt;Active&lt;/option&gt;
        &lt;option value="opt2" label="inactive"&gt;inactive&lt;/option&gt;
        &lt;option value="opt3" label="terminated"&gt;terminated&lt;/option&gt;
   &lt;/select&gt;
  </code></pre>
  
  <p>So why does it add string: to the value attribute? How i can get my desired output?</p>