title: How to use ion-radio with formControlName
body: <p>I'm using ion-radio compoenent of ionic 4 in my projetct I want to set the visiteur option selected by default  I put [checked]="true" but it doesnt work:</p>
  
  <pre><code> &lt;ion-list style="margin-top: 5px;" class="ion-padding"&gt;
          &lt;ion-radio-group  (ionChange)="onChangeHandler($event)"  formControlName="userType"&gt;
  
  
  
            &lt;ion-item&gt;
              &lt;ion-label color="penblue"&gt;Entreprise&lt;/ion-label&gt;
              &lt;ion-radio color="penbluegrig" slot="start" value="Entreprise"&gt;&lt;/ion-radio&gt;
            &lt;/ion-item&gt;
  
            &lt;ion-item&gt;
              &lt;ion-label color="penblue"&gt;Transporteur&lt;/ion-label&gt;
              &lt;ion-radio color="penbluegrig"  slot="start" value="Transporteur"&gt;&lt;/ion-radio&gt;
            &lt;/ion-item&gt;
  
            &lt;ion-item&gt;
              &lt;ion-label color="penblue"&gt;Visiteur&lt;/ion-label&gt;
              &lt;ion-radio color="penbluegrig"  slot="start" [checked]='true'  value="Visiteur"&gt;&lt;/ion-radio&gt;
            &lt;/ion-item&gt;
  
          &lt;/ion-radio-group&gt;
        &lt;/ion-list&gt;
  </code></pre>
  
  <p>How to resolve this problem.? please?</p>