title: Binding to [value] doesn't update FormControl's value
body: <p><strong>More Info:</strong></p>
  
  <p>I have a list of items. Each item is an instance of a <code>MyClass</code>. <code>MyClass</code> has a property <code>text</code> and a method <code>someMethod()</code> that returns this property.</p>
  
  <p>I have an <code>*ngFor</code> that lists all items on the page and when I click one of them <code>(click)="currentItem = item"</code> is executed and the <code>[value]="someMethod()"</code> is actually <code>[value]="currentItem.someMethod()"</code>, so the selected item's text is written into the textarea.</p>
  
  <hr>
  
  <p>I'm using Reactive forms and have the following in my template:</p>
  
  <pre><code>&lt;textarea formControlName="myTextarea" [value]="someMethod()" ...
  </code></pre>
  
  <p>When the page renders and <code>someMethod()</code> outputs a string, I can see that string displaying inside the textarea.</p>
  
  <p>However, the <code>value</code> property of the <code>FormControl</code> is not updated. Why?</p>