title: Primeng expandable column headers don't show
body: <p>I have an expandable row in Angular2 using Primeng2 but the column headers for the expandable columns aren't showing.</p>
  
  <p>This is my table with the expandable rows:</p>
  
  <pre><code>   &lt;p-dataTable [value]="activetrucks" expandableRows="true" [(selection)]="selectedtruck"&gt;
    &lt;p-header&gt;
      List of Active trucks
  
    &lt;/p-header&gt;
    &lt;p-column expander="true"&gt;&lt;/p-column&gt;
    &lt;p-column field="owner_id" header="Transporter"&gt;&lt;/p-column&gt;
    &lt;p-column field="reg_no" header="Truck number"&gt;&lt;/p-column&gt;
    &lt;p-column field="truck_category" header="Truck type" &gt;&lt;/p-column&gt;
  
    &lt;template let-user pTemplate="rowexpansion"&gt;  //headers dont show
      &lt;div class="ui-grid ui-grid-responsive ui-fluid"&gt; 
        &lt;p-dataTable [value]="stages" [paginator]="true" [pageLinks]="3"&gt;
          &lt;p-header&gt;
            The different stages
          &lt;/p-header&gt;
          &lt;p-column field="status" header="Status"&gt;&lt;/p-column&gt;  
          &lt;p-column field="created_at" header="Done by"&gt;&lt;/p-column&gt;
          &lt;p-column field="created_by" header="Date"&gt;&lt;/p-column&gt;
  
          &lt;p-column styleClass="col-button"&gt;
            &lt;template let-truck="rowData" pTemplate="body"&gt;
              &lt;button type="button" pButton (click)="onremoveTruck(truck)" icon="fa-remove"&gt;&lt;/button&gt;
            &lt;/template&gt;
          &lt;/p-column&gt;
  
        &lt;/p-dataTable&gt;
      &lt;/div&gt;
    &lt;/template&gt;
  
  &lt;/p-dataTable&gt;
  </code></pre>
  
  <p>In the expand row the headers dont show as shown in picture below.</p>
  
  <p><a href="https://i.stack.imgur.com/GONYV.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/GONYV.png" alt="enter image description here"></a></p>
  
  <p>What could be wrong?</p>