CharsetConverter::convert should not try to convert numeric value

[delmicio]

Issue summary

After updating from 9.1.2 to 9.1.3 my code is no longer working without error.

System informations

Information	Description
League\Csv	9.1.3
PHP	7.1.12
ubuntu	16.04

Standalone code, or other way to reproduce the problem

$formatter = (new Csv\CharsetConverter())
    ->inputEncoding('utf-8')
    ->outputEncoding('Windows-1252');
$csvWriter = Csv\Writer::createFromFileObject(new \SplFileObject('php://output', 'w'));
$csvWriter->addFormatter($formatter);
$csvWriter->insertOne(['str', '1', 2]); 

Expected result

No errors.

Actual result

Type error: mb_convert_encoding() expects parameter 1 to be string, integer given at /var/www/vendor/league/csv/src/CharsetConverter.php:217

[nyamsprod]

Bug fixed in the master branch .. a patch version will be released later this week with the bug fix.