title: angular ng-model is not working for input type=checkbox
body: <p>My code is like this:
  html:</p>
  
  <pre><code>&lt;input type="checkbox" ng-model="cb1"&gt; You have discussed the amount, frequency and term applicable to the payment schedule with your customer and they have agreed.
  </code></pre>
  
  <p>get the checkbox value with</p>
  
  <pre><code>console.log('checkbox:', $scope.cb1);
  </code></pre>
  
  <p>the result is always <code>undefined</code> regards whether I check the box or not.</p>
  
  <p>But if my code changes to</p>
  
  <pre><code>&lt;input type="checkbox" ng-model="cb1.value"&gt; You have discussed the amount, frequency and term applicable to the payment schedule with your customer and they have agreed.
  </code></pre>
  
  <p>and initialize the variable:</p>
  
  <pre><code>$scope.cb1 = {value: true};
  </code></pre>
  
  <p>then I can get the checkbox value with</p>
  
  <pre><code>console.log('checkbox:', $scope.cb1.value);
  </code></pre>
  
  <p>would anybody tell me why? thanks</p>