If debug is enabled, ask_for_optional is a required field

I was going though [this](http://blog.miguelgrinberg.com/post/the-flask-mega-tutorial-part-v-user-logins) and noticed that when `__debug__` is true I had to include `ask_for_optional` in the `try_login` function else it would break. Just included an `if ask_for_optional:` to fix this.