title: The value of primeng's slider doesn't updates
body: <p>I've a simple component with a slider and two buttons:</p>
  
  <p><strong>app.component.html</strong></p>
  
  <pre><code>&lt;p-slider [(ngModel)]="val" [min]="0" [max]="100" [step]="1"&gt;&lt;/p-slider&gt;
  
  &lt;button pButton type="button" label="Next" (click)="next()"&gt;&lt;/button&gt;
  &lt;button pButton type="button" label="Reset" (click)="reset()"class="ui-button-secondary"&gt;&lt;/button&gt;
  </code></pre>
  
  <p><strong>app.component.ts</strong></p>
  
  <pre><code>import { Component } from '@angular/core';
  import { SliderService } from './slider.service';
  
  @Component({
    selector: 'my-app',
    templateUrl: './app.component.html',
    styleUrls: [ './app.component.css' ]
  })
  export class AppComponent  {
    val: number;
  
    next() {
      this.val++;
    }
  
    reset() {
      this.val = 0;
    }
  }
  </code></pre>
  
  <p>The problem is, when I click on <code>Next</code> button, it does nothing. It should increment the sliders value.</p>
  
  <p>Stackblitz <a href="https://stackblitz.com/edit/primeng-template-rouffe?file=app/app.component.ts" rel="nofollow noreferrer">link</a>.</p>