Fix name({prefix:true}) always generating gender-neutral prefixes

The generated gender, passed to `name_prefixes` would not be all lower-case, this causes only "Dr." to be used as an available prefix.

This problem could also be fixed by converting the gender in `name()` to lower case, but I chose to fix it in `name_prefixes` to also allow different casing for `gender` to be supplied in that generator.

Fixes #117