title: Drop Down Value Not Returning To Default ng-if
body: <p>I have the below angular form, using ng-if the drop downs appear when a radio button is clicked. When a different radio button is clicked again the drop downs go to a blank value, I would like them to go back to the "Choose" option. How do I do that?</p>
  
  <pre><code>&lt;form name="addToolForm" ng-submit="vm.addStuff()"&gt;
      &lt;div layout="row" style="padding-left:30px"&gt;
          &lt;md-radio-group name="firstCategory" ng-model="vm.firstCategory" layout="row" ng-required="true"&gt;
              &lt;md-radio-button value="item1"&gt;item1&lt;/md-radio-button&gt;
              &lt;md-radio-button value="item2"&gt;item2&lt;/md-radio-button&gt;
          &lt;/md-radio-group&gt;
      &lt;/div&gt;
      &lt;br&gt;&lt;br&gt;
      &lt;div class="col-xs-12" id="secondCategory" style="padding-left:30px" ng-if="vm.firstCategory == 'item1'"&gt;
          &lt;label class="col-xs-6 control-label"&gt;Sub Type:&lt;/label&gt;
          &lt;div class="col-xs-6"&gt;
              &lt;select name="secondCategory" ng-model="vm.secondCategory" ng-dropdown required="true"&gt;
                  &lt;option value="" disabled selected&gt;Choose&lt;/option&gt;
                  &lt;option ng-option value="option1"&gt;option1&lt;/option&gt;
                  &lt;option ng-option value="option2"&gt;option2&lt;/option&gt;
              &lt;/select&gt;
          &lt;/div&gt;
      &lt;/div&gt;
      &lt;div class="col-xs-12" id="secondCategory" style="padding-left:30px" ng-if="vm.firstCategory == 'item2'"&gt;
          &lt;label class="col-xs-6 control-label"&gt;Sub Type:&lt;/label&gt;
          &lt;div class="col-xs-6"&gt;
              &lt;select name="secondCategory" ng-model="vm.secondCategory" ng-dropdown required="true"&gt;
                  &lt;option value="" disabled selected&gt;Choose&lt;/option&gt;
                  &lt;option ng-option value="option1"&gt;option3&lt;/option&gt;
                  &lt;option ng-option value="option2"&gt;option4&lt;/option&gt;
              &lt;/select&gt;
          &lt;/div&gt;
      &lt;/div&gt;
  &lt;/form&gt;
  </code></pre>