title: "[(ngModel)]" not binding in Ionic 4
body: <p>I'm building an Ionic application, currently migrating from Ionic 3 to 4. I've got some code which works perfectly in Ionic 3, but not in 4.
  Basically, I can't seem to bind data with [(ngModel)] in an input field. I've imported <code>FormsModule</code> and added it among my imports in my <code>app.module.ts</code>. Here's how my <code>-component.ts</code> file looks like:</p>
  
  <pre><code>export class SomeComponent implements OnInit {
    someString: string = "test";
    ...
  }
  </code></pre>
  
  <p>And my <code>-component.html</code></p>
  
  <pre><code>&lt;ion-input type="text" [(ngModel)]="someString"&gt;&lt;/ion-input&gt;
  {{someString}}
  </code></pre>
  
  <p>Now in my application, <code>{{someString}}</code> rightly shows "test". However, changing the value of the input doesn't affect the <code>someString</code> variable in any way. What could I be missing?</p>