title: Why Value Formatter gets Applied to the Cell but the Displayed Value does Not Change ag-grid
body: <p>I do have the following <em>column</em> and <em>function</em> in my ag-grid:</p>
  
  <pre><code>  ngOnInit(): void {
  
              this.columnDefs = [
                  {
                  headerName: 'Header', field: 'quantity',
                  valueFormatter: (params) =&gt; this.numberFormatter(params),
                  }
          ];
     }
  
    numberFormatter(params){
      console.log(params.value);
      return '$'+params.value;
    }
  </code></pre>
  
  <p>No matter what changes make to the current value <strong>inside</strong> the <em>numberFormatter</em>() function, no changes are displayed in the table. </p>
  
  <p>The changes however are visible and <strong>returned</strong> <strong>correctly</strong> within <em>numberFormatter</em>() method.</p>
  
  <p>What am I missing?</p>