Signing with undefined secret never gets resolved #286

Currently there is a bug in node-jws where supplying undefined or other falsy value to secret results in the signing process never running to completion.

The solution for the problem is known and should probably be fixed upstream, but since this directly affects this library as well and given its popularity I thought I'd let you know. Perhaps you'd like to make sure secret has a value before handing it over to JWS for signing, I do not see much use of using undefined as my secret, anyway.

Good luck and merry Christmas! 🎁 🎅