make sure :sub check behaves like :aud check

[skippy]

if :verify_sub is turned on and options includes a :sub, make sure we
fail if the payload is not set or doesn't match. Previously if the
payload did not have 'sub' set, it would not verify, even if verify_sub
was true

see #102

[skippy]

I was tempted to also add a check where if :verify_sub is set but options[:sub] is not set, that an error would be thrown. That may change the behavior a bit too much, esp. for the scope of this bug.

But in general, if :verify is set, I would expect it to fail if :sub or :aud is not passed in. If you would like me to, I can open another issue to track that edge case.

thanks!