Properly standardize RFC1751 inputs

While spelunking through Ripple code (instead of sleeping) I stumbled across `RFC1751::standard`. The intention of the code seems to be to convert the input string to all uppercase ASCII characters and replace the numerals `0`, `1` and `5` with the similar-looking ASCII characters `'O'`, `'I'` and `'S'`.

Unfortunately, the function was a no-op: it modified the local copy of the variable `cLetter`, leaving the original string unchanged.

I believe this change properly reflects the intention of the code.