title: sinon - callsArgWith advanced usage
body: <p>I can use <code>callsArgWith</code> to test callbacks, for example:</p>
  
  <pre><code>var modelsMock = { 
    Users: {
      findOne: sinon.stub()
    }
  };
  
  usersController.__set__({
    'models': modelsMock, 
  }); 
  
  //... inside a test:
  var user = 'foo';
  modelsMock.Users.findOne.callsArgWith(1, true, user);
  </code></pre>
  
  <p>this works just fine when my code is:</p>
  
  <pre><code>models.Users.findOne(id, function(err, user) {
     //stuff here.
  });
  </code></pre>
  
  <p>but how can I test a callback in the following situation?</p>
  
  <pre><code>var user = new models.Users(userData);
  user.save(function(err) {
    //I want to test this callback
  });
  </code></pre>
  
  <p>Let me know if it is not clear enough and I will try to explain better.</p>