title: Material-table CustomSort doesn't work with Dates
body: <p>I receive a column from api that contains Dates + _number (example: "20/12/2019_1" as a string). The sort function will group counting data as a string and not as a Date column. Which means i need to do a customSort.
  So i created a function to transform that string into a Date object, but when i return the value nothing happens...</p>
  
  <p><strong>Columns:</strong></p>
  
  <p><img src="https://i.stack.imgur.com/Tik3C.png" alt="Columns"></p>
  
  <p><strong>utils.js:</strong></p>
  
  <p><img src="https://i.stack.imgur.com/97GaS.png" alt="utils.js"></p>
  
  <p>The formatLot function is working as expected since i wanna Sort value as type Date. </p>
  
  <p><strong>Result from formatLot function:</strong></p>
  
  <p><img src="https://i.stack.imgur.com/GqTcv.png" alt="Result from formatLot function"></p>