Verifying expiration without leeway throws exception

[fightingtheboss]

Both in 1.5.6 and in master, trying to verify the exp of a token without providing an explicit leeway throws the following exception:

TypeError: nil can't be coerced into Integer

This is because this line (on master) relies on the return value from the options Hash to add time to the exp integer. If leeway isn't set in the options, the above error is thrown.

I think the leeway should default to 0 if not found, as seems to be the assumption in the specs.