An error poped out when user register

[simonmatt]

Error message: The "digest" argument must be one of type string or null. Received type undefined

Solution:
Update [./utility/hash.js]

exports.hash = function (pwd, salt, fn) {
    if (3 == arguments.length) {
        crypto.pbkdf2(pwd, salt, iterations, len, **null**, function (err, hash) {
            fn(err, hash.toString('base64'));
        });
    } else {
        fn = salt;
        crypto.randomBytes(len, function (err, salt) {
            if (err) {
                return fn(err);
            }
            salt = salt.toString('base64');
            crypto.pbkdf2(pwd, salt, iterations, len, **null**, function (err, hash) {
                if (err) return fn(err);
                fn(null, salt, hash.toString('base64'));
            });
        });
    }
};

Added a null value as default value of 'hex' variable