Classes stored as string but treated like array when creating a new field group

On lines 635 and 636 of field_group.field_ui.inc:

    $classes = _field_group_get_html_classes($new_group);
    $new_group->format_settings['instance_settings']['classes'] = $classes->optional;

Looking at _field_group_get_html_classes(), it's clear that $classes->optional is an array.

However, $new_group->format_settings['instance_settings']['classes'] expects a string (no special treatment in submit callback for editing field groups).

Also, field_group_format_settings_summary() expects a string value and check_plain() chokes on the array.

Implementing hook_field_group_format_summary() doesn't allow us to fix that either as it only appends content to the summary and doesn't allow altering it.