fixes token_lifetime with nil values issue

[muZk]

You can't have multiple entities where one of them has permanent tokens. This configuration would fail:

      Knock.token_lifetime = {
        user: nil,
        admin: 7.days,
      }

Because token_lifetime method assumes that if this is a hash, the values are dates/datetime:

def token_lifetime
      return unless verify_lifetime?

      if Knock.token_lifetime.is_a?(Hash)
        Knock.token_lifetime[entity_class_name].from_now.to_i // <---- here
      else
        Knock.token_lifetime.from_now.to_i
      end
end

This PR fixes the issue by updating verify_lifetime? method.