XML Parser - multiple non-string values reduced to a single value

If a direct child of the item_selector has multiple values and those values are not strings, only a single value is preserved and passed to any process plugins.

From Drupal\migrate_plus\Plugin\migrate_plus\data_parser\Xml::fetchNextRow() ~ line 249

// If the SimpleXMLElement doesn't render to a string of any sort,
// and has children then return the whole object for the process
// plugin or other row manipulation.
if ($value->children() && !trim((string) $value)) {
  $this->currentItem[$field_name] = $value; // The culprit
}
else {
  $this->currentItem[$field_name][] = (string) $value;
}

Fix:

// If the SimpleXMLElement doesn't render to a string of any sort,
// and has children then return the whole object for the process
// plugin or other row manipulation.
if ($value->children() && !trim((string) $value)) {
  $this->currentItem[$field_name][] = $value; // Match the else, preserve all values.
}
else {
  $this->currentItem[$field_name][] = (string) $value;
}

Source XML:

<Categories>
  <Category>
      <Code>category_1</Code>
      <Name>Category 1</Name>
      <Items1>
        <SubItem>
          <Id>1</Id>
          <Name>Name 1</Name>
        </SubItem>
        <SubItem>
            <Id>2</Id>
          <Name>Name 2</Name>
        </SubItem>
      </Items1>
      <Items2>
          <SubItem>1</SubItem>
          <SubItem>2</SubItem>
      </Items2>
  </Category>
</Categories>

Migration template:

id: test_migration
migration_group: test
source:
  plugin: url
  urls: 'private://categories.xml'
  data_fetcher_plugin: file
  data_parser_plugin: xml
  item_selector: /Categories/Category

  ids:
    code:
      type: string

  fields:
    -
      name: code
      label: 'Code'
      selector: 'Code'
    -
      name: name
      label: 'Name'
      selector: 'Name'
    -
      name: sub_items1
      label: 'Sub items 1'
      selector: 'Items1/SubItem'
    -
      name: sub_items2
      label: 'Sub items 2'
      selector: 'Items2/SubItem'

process:
  vid:
    plugin: default_value
    default_value: test

  name: name

destination:
  plugin: entity:taxonomy_term

For fields which have one level selector (like sub_items2) it works fine. But for fields with several children and complex structure like sub_items1 it's returning only last element.