message: Added the `getItemValueCallback` for the
diff: @@ -70,6 +70,7 @@ export class ChoicesRestfull extends Base {
  items: Array<ItemValue>,
  serverResult: any
  ) => Array<ItemValue>;
  + public getItemValueCallback: (item: any) => any;
  public error: SurveyError = null;
  public owner: IQuestion;
  constructor() {
  @@ -283,7 +284,7 @@ export class ChoicesRestfull extends Base {
  for (var i = 0; i < updatedResult.length; i++) {
  var itemValue = updatedResult[i];
  if (!itemValue) continue;
  - var value = this.getValue(itemValue);
  + var value = !!this.getItemValueCallback ? this.getItemValueCallback(itemValue) : this.getValue(itemValue);
  var title = this.getTitle(itemValue);
  var item = new ItemValue(value, title);
  this.setCustomProperties(item, itemValue);
  