nbf claim not used when passed on the payload (solve #453)

[JacopoPatroclo]

Use nbf claim if present on payload.

closes #453

[simoneb]

Would it make sense to include tests that also decode the token and check that the right value of the field is in the payload, according to the rules we have in place?

[JacopoPatroclo]

I thought about doing that but then I saw that the project follow a different structure for the tests separating sign and verification procedures I've change approach.
I will create a new suite to test the signing process back and forth for a couple of standard cases.

[simoneb]

@JacopoPatroclo it was just a suggestion, I didn't actually check what's in the codebase, I just thought about it when looking at the existing tests and seeing they assert against an encoded token, which is hard to make sense of when looking at the code