title: HTML checkbox with [checked]="model?.value" returns null when assigned to a model property
body: <p>I have this issue where in ngOnInit I preform a http GET for record data to fill a form. This form is essentially a page that allows the user to edit and save information back to the database. The issue that I am running into is that if the user has filled out the form and checked this checkbox in say a previous PUT obviously the model.BooleanValue = true and the checkbox is checked when the page is initialized.</p>
  
  <pre><code>&lt;input type="checkbox" formControlName="isBoolean" [checked]="model?.BooleanValue" /&gt;
  </code></pre>
  
  <p>However, lets assume I did not make any changes to the checkbox (the checkbox is still checked from the http GET) and I preform a PUT (without editing the checkbox in any way). When I assign the form checkbox value to the model.BooleanValue the checkbox value/checked returns null rather than true.</p>
  
  <p>Here is the FormToModel method that I am using as well as the model. (Lets assume that the ngOnInit - GET worked and appropriately assigned the checkbox to checked and the model.Boolean value is true)</p>
  
  <pre><code>export class BoolModel { 
      public BooleanValue: Boolean;
  }
  
  FormToModel(f: FormGroup) {
      var model = new BoolModel();
      model.BooleanValue = f.controls['isBoolean'].value ? true : false;
      return model;
  }
  </code></pre>
  
  <p>The GET and PUT work just fine and the only problem is when I attempt to assign model.BooleanValue = isBoolean.value during the PUT (This causes the isBoolean value to be null and thus assigned false which is clearly not the boolean value of the checkbox visually in the form or in the model).</p>
  
  <p>Also note: If I uncheck and recheck the checkbox the value will pass as true and not null. So, it seems that the checkbox needs to be interacted with in order to send a correct value.</p>
  
  <p>Its probably something simple to fix but I cannot seem to make it work... Any insight would be greatly appreciated.</p>