Fixing ArrayAdapter optgroup children data binding

[5andr0]

ArrayAdapter.convertToOptions fails to create item data for optgroup children, not binding it to the select options.
This results in a bug, where the passed data element to select(data) is not initialized and normalized, thus element is null and id is an integer. Therefore the select function can't find the corresponding <option> while comparing an integer id with string <option>[value] and creates duplicate options when selecting optgroup child options, which was meant for adding search tags:

ArrayAdapter.prototype.select = function (data) {
  var $option = this.$element.find('option').filter(function (i, elm) {
    return elm.value == data.id.toString();
  });

  if ($option.length === 0) {
    $option = this.option(data);

    this.addOptions($option);
  }

Here's a polyfill for the bugfix

$.fn.select2.amd.require(['select2/data/array'],
  function (ArrayAdapter) {
    let __super__convertToOptions = ArrayAdapter.prototype.convertToOptions;
    ArrayAdapter.prototype.convertToOptions = function (data) {
      var self = this;

      const $options = __super__convertToOptions.call(this, data);
      for(const $option of $options) {
        (function applyChildData(option) {
          if (option.children) {
            option.children.forEach(function(child, index, children) {
              var $child = $($(option.element).children()[index]);
              children[index] = self.item($child);
              applyChildData(children[index]);
            });
          }
        })(self.item($option));
      }
      return $options;
    }

    ArrayAdapter.prototype.select = function (data) {
      if (!data.element) {
        this.addOptions(this.option(data));
      }
  
      ArrayAdapter.__super__.select.call(this, data);
    };
  }, null, true);

This pull request includes a

• Bug fix
• New feature
• Translation

The following changes were made

• Creating a data cache for each optgroup child (recursively for nested optgroups) and binding it correctly to its corresponding select option
• simplifying the search if there exists an option corresponding to the data passed to select, also making it available for optgroups and not only options.