title: Exporting to excel with proper cell formatting on ag-grid
body: <p>When I export a table to excel using ag-grid's own <code>exportDataAsExcel()</code> the resulting excel contains dates as <strong>General</strong> data type instead of <strong>Date</strong>. </p>
  
  <p>[<img src="https://i.stack.imgur.com/XyVMp.png" alt="excel pic">]</p>
  
  <p>I have used this:</p>
  
  <pre><code>exportDataAsExcel({
     processCellCallback: ({col, val}) =&gt; {( /*date value formatting here*/ )}
  })
  </code></pre>
  
  <p>to format both <code>Date</code>, <code>string</code> with proper date formatting <code>(DD/MM/YYYY)</code> but I can't make excel properly recognize these cells as <strong>Date</strong> instead of <strong>General</strong></p>
  
  <p>This is reproducible with the excel export examples on their website: <a href="https://www.ag-grid.com/javascript-grid-excel/?framework=all#gsc.tab=0" rel="noreferrer">https://www.ag-grid.com/javascript-grid-excel/?framework=all#gsc.tab=0</a></p>