title: Angular Material 2 DataSource filter with nested object
body: <p>I have angular 4 project with material 2, I want to filter the data in MatTable. DataSource filter is working fine when we filter data on field which are not nested. </p>
  
  <pre><code>this.dataSource = new MatTableDataSource([
       { orderNumber: 1, orderInfo: { type: 'ABC'}, date: '12/3/2012 9:42:39 AM'},
       { orderNumber: 3, orderInfo: { type: 'Hello' }, date: '12/2/2018 9:42:39 AM'},
  ]);
  </code></pre>
  
  <p>Filter is working fine for orderNumber, date but not working properly with type field in orderInfo object.</p>