title: PrimeNG Treetable adding nodes issue
body: <p>I'm using latest stable versions of Angular (6.0.9) and PrimeNG (6.0.1). When I try to add child node to selected node programmatically it doesn't show up unless i collapse and then expand selected parent node. First i tried</p>
  
  <pre><code>this.selectedNode.children.push(this.nodeToAdd);
  </code></pre>
  
  <p>Then I saw in official documentation that if "<em>you manipulate the value such as removing a node, adding a node or changing children of a node, instead of using array methods such as push, splice create a new array reference using a spread operator or similar</em>". So i tried this:</p>
  
  <pre><code>this.selectedNode.children = [...this.selectedNode.children, this.nodeToAdd];
  </code></pre>
  
  <p>but it results in same behavior. Does anyone facing same issue, or am I doing something wrong? Issue is happening in Chrome, Firefox and Edge</p>