title: ng2-smart-table with custom editor, not updating datasource on edit
body: <p>I have an issue with ng2-smart-table, which I fill with an empty localdatasource.</p>
  
  <p>I have a Date column, which use a custom editor : </p>
  
  <pre><code>dateInscription: {
                  title: 'Date',
                  filter:false,
                  type: 'string',
                  valuePrepareFunction: (cell, row) =&gt; {
                      return `${new Date(cell).toLocaleDateString()}`;
                  },
                  editor: {
                      type: 'custom',
                      component: DateRenderComponent,
                  },
              }
  </code></pre>
  
  <p>The editor extends DefaultEditor and of course open a datepicker (p-calendar). When the user select a date, the cell value is updated using :</p>
  
  <pre><code>onSelect(event)
      {
        this.cell.newValue = this.value;
      }
  </code></pre>
  
  <p>The event is well fired. This work in new rows... What could be wrong ?</p>
  
  <p>Once the row is created, I want to edit it, and this don't work. When I click the "save" command, the old value stay (note that other simple text columns are updated).</p>
  
  <p>I can see the valuePrepareFunction beeing called after clicking "save", with the old cell value.</p>
  
  <p>I've double check than the onSelect event is setting the good value in cell.newValue</p>
  
  <p>Did someone already got this issue ? What was the solution ?
  Thanks !</p>