title: Angular - formGroupName value change does not update the form
body: <p>As described in the title, <code>formGroupName</code> value change does not update the form. 
  (after hitting the button the value displayed in the input remains the same)</p>
  
  <p>Here is a <a href="https://plnkr.co/edit/VTOMy4WCvuZMJAsYrULz?p=preview" rel="noreferrer">plunker</a>.</p>
  
  <pre><code>@Component({
    selector: 'my-app',
    template: `
     &lt;form [formGroup]="myForm"&gt;
      &lt;div [formGroupName]="fgn"&gt;
          &lt;input [formControlName]="'name'"&gt;
      &lt;/div&gt;
  &lt;/form&gt;
  
  &lt;button (click)="formChange()" &gt;Change&lt;/button&gt;
  
  {{fgn}}
  &lt;br&gt;
  {{myForm.value | json}}
    `,
  })
  export class App {
  
      private myForm: FormGroup;
      private fgn: String;
  
      ngOnInit() {
          this.fgn = 'zero';
  
          this.myForm = this.formBuilder.group({
              zero: this.formBuilder.group({
                  name: 'Zero'
              }),
              one: this.formBuilder.group({
                  name: 'One'
              })
          });
      }
  
      formChange() {
          this.fgn = 'one';
      }
  
      constructor(private formBuilder: FormBuilder) {
  
      }
  }
  </code></pre>