Add UTF-8 encoding to CsvWriter

When creating a csv file, UTF-8 encoded strings aren't supported in Excel without "chr(0xEF) . chr(0xBB) . chr(0xBF)" at the beginning of the file.

Example with french words : 

    $writer = new CsvWriter(';', '"', null, true);
    $writer->writeItem(array('Précédent', 'Suivant'));
    $writer->finish();

Question : In the CsvWriter constructor, why is the stream in the third position? In ArrayWriter and ExcelWriter the output is the first parameter.