title: The value from a mat-option disappear when I edit another property
body: <p>I'm using Angular 6 and Material Angular in my project. If I edit one of the properties other than 'tipo', when I submit the form, the value of 'tipo' disappears. But when I edit only the 'tipo' property, this does not happen. How can I fix this? Thank you.</p>
  
  <p>HTML file:</p>
  
  <pre><code>&lt;form [formGroup]="updateForm" class="edit-form"&gt;
    &lt;mat-form-field&gt;
      &lt;mat-select placeholder="Tipo" formControlName="tipo" #tipo&gt;
        &lt;mat-option value="Conta Corrente"&gt;Conta Corrente&lt;/mat-option&gt;
        &lt;mat-option value="Conta Poupança"&gt;Conta Poupança&lt;/mat-option&gt;
        &lt;mat-option value="Caixinha"&gt;Caixinha&lt;/mat-option&gt;
      &lt;/mat-select&gt;
    &lt;/mat-form-field&gt;
    &lt;button type="submit" (click)="updateConta(tipo.value, identificacao.value, saldoInicial.value, agencia.value, cc.value)" [disabled]="updateForm.pristine || updateForm.invalid" mat-raised-button color="primary"&gt;Save&lt;/button&gt;
  &lt;/form&gt;
  </code></pre>
  
  <p>TS file:</p>
  
  <pre><code>  updateConta(tipo, identificacao, saldoInicial, agencia, cc) {
      this.contaService.update(this.id, tipo, identificacao, saldoInicial, agencia, cc).subscribe(() =&gt; {
        this.snackBar.open('Conta atualizada com sucesso', 'OK', {
          duration: 3000
        });
        this.goBack();
      });
    }
  
    goBack() {
      this.router.navigate(['/contas']);
    }
  </code></pre>