missing separator-characters when writing a single value to a multi-select-field using the api

The api-explorer passes a single value for a multi-select-field as string. But CRM_Core_BAO_CustomField::formatCustomField expects values for multi-select-fields to be an array (or when merging contacts an already prepared string).
At least the value will be written to the database without any separators and that will lead to some problems when editing the entity.