title: ngAnimateSwap - using boolean expression does not animate as expected
body: <p>I modified the original ngAnimateSwap example this is found in the docs <a href="https://docs.angularjs.org/api/ngAnimate/directive/ngAnimateSwap%20ngAnimateSwap" rel="nofollow">enter link description here</a> to use a boolean expression instead of an integer to trigger the slide animation. </p>
  
  <p>I would expect the banner to rotate between 'true' and 'false' however instead, only the 'true' appears, but the 'false' does not. From the angular docs: "ngAnimateSwap is a animation-oriented directive that allows for the container to be removed and entered in whenever the associated expression changes. " so I would have expected changing between true and false would cause the container to animate.</p>
  
  <p>In the html:</p>
  
  <pre><code>  &lt;div class="container" ng-controller="AppCtrl"&gt;
      &lt;div ng-animate-swap="abool" class="cell swap-animation" ng-class="colorClass(abool)"&gt;
        {{ abool }}
      &lt;/div&gt;
    &lt;/div&gt;
  </code></pre>
  
  <p>In the controller:</p>
  
  <pre><code>$scope.abool = false;
  $interval(function() {
    $scope.abool = !$scope.abool
  }, 1000);
  
  $scope.colorClass = function(abool) {
    return abool ? 'red' : 'blue';
  };
  </code></pre>
  
  <p>Here is the plunkr showing the example:
  <a href="http://plnkr.co/edit/iKE5BekgpxOqX1YE952Z?p=preview" rel="nofollow">http://plnkr.co/edit/iKE5BekgpxOqX1YE952Z?p=preview</a></p>
  
  <p>As another test, if you update to my plunkr to change <code>abool</code> to toggle between 1 and -1, then the animation looks as expected - sliding between 1 and -1.</p>