fix: reply.jwtSign sets wrong exp

[maticjeras]

When using reply.jwtSign without a callback convertTemporealProps gets called two times multiplying time props twice incorrectly.

• move the check for the callback before the call to convertTemporalProps, so they are not called twice
• add assertions checking token exp prop to existing time definitions tests

Closes #207

Checklist

• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

[climba03003]

LGTM.

[mcollina]

lgtm