Bugfix for long random strings and more randomness

Bugfix to allow long random strings (> 5*26) and more randomness (string with length=10 must allow 10 times the same chars)

this method does not claim to bring security nor anything in crypto, it is only usefull to manage strings. Maybe if should be removed from laravel, but if inside, I think it should do correctly things is has to do.
@mayoz, your proposition does not solve the problem and may have incoherent behavior if length > 26*repeat.