title: Question mark in AngularJs directive
body: <p>I have seen some directive using question mark ('?') in the scope properties definition like so:</p>
  
  <pre><code>    app.directive('userInfo', function() {
          return {
          restrict: 'A',
          scope: {prop: '=?'},
          templateUrl: 'some/template/url',
      };
  </code></pre>
  
  <p>I have looked for it and the only think i saw was this : <a href="https://stackoverflow.com/questions/20447786/whats-the-meaning-of-in-angularjs-directive-isolate-scope-declaration">what&#39;s the meaning of &#39;=?&#39; in angularJS directive isolate scope declaration?</a>. But it's not working the way he explains it there. I have succeeded running my application and no Exception was thrown.</p>
  
  <p>Can somebody elaborate more about it?</p>