problem if the string is passed as int

[91DarioDev]

https://github.com/lepture/captcha/blob/fb6238e741c7e264eba117b27fa911c25c76c527/captcha/image.py#L196

if the string is passed as int, the function goes in error cause it tries to iterate an integer. an easy fix is to force the cast as string doing for str(c) ..