title: Multiple row selection with datakey
body: <p>I'm working on a project that used PrimeNg as UI library. 
  While working with the table, I found that the datakey property does not select all the related rows. When I console log the selection array, it shows only 1 was selected. But the UI selects all the related rows. </p>
  
  <p>For example, I have a table that contains row with multiple trackingNo. 
  Every row might have the same trackingNo. 
  When I select one of the row, it should select all the related rows with the same trackingNo. </p>
  
  <p>Am I missing something? Or is it a bug ?</p>
  
  <pre class="lang-html prettyprint-override"><code>    &lt;p-table 
                       [(selection)]="selectedRows"
                       [columns]="cols"
                       [dataKey]="'trackingNo'"
                       [selectionMode]="'multiple'"
                       [lazy]="false"
                       [rows]="20"
                       [value]="values$ | async"&gt;
  </code></pre>