Fix exception when types are not coherent

[loevgaard]

I got this exception:

Uncaught TypeError: str_repeat() expects parameter 2 to be integer, float given in /Users/loevgaard/PhpstormProjects/SyliusStockMovementPlugin/vendor/sebastian/exporter/src/Exporter.php:255

and fixed it by doing this small change.

[sebastianbergmann]

I would like to understand why it is float instead of int before (possibly) hiding any underlying root cause.