Salt generated as base64, but interpreted as UTF-8

The ```Salt.generateAsBase64String``` function produces a string that is the base64 representation of a random byte sequence. The ```PBKDF2.generateKey``` function takes the salt as a string but interprets it as a UTF-8 string. Since the salt is actually a base64 string, and not a UTF-8 string, there is some loss in the available range.