autoGenerateGetSet() => set() => break if beforeSet has no return value

see #242 (Nige)

            if (me[beforeSet] && typeof me[beforeSet] === 'function') {
                value = me[beforeSet](value, oldValue);

                // If they don't return a value, that means no change
                if (value === undefined) {
                    return;
                }
            }

The idea is eg

beforeSetStartDate(newDate, oldDate) {
    if (typeof newDate === 'string') {
        newDate = DateHelper.parse('YYYY-MM-DD');
    }

    // Only return a value if it's a *valid* value (!isNaN) and is actually changed.
    // undefined return value means the change is vetoed - the calling code returns early.
    if (!isNaN(newDate) && (!oldDate || oldDate.valueOf() !== newDate.valueOf())) {
        return newDate;
    }
}

Without the change, each beforeSet method with if conditions would need something like

else {
    return oldValue;
}

so it might shorten the app code a little bit. I will give it a try, although return undefined; might cause issues in case it ever happens.