Take time skew into consideration

[klippx]

We have seen instances where expired? returns false but it actually is expired, and we think it might be due to time skew.

So, when checking if a token is expired, we should try to take time skew into consideration.

Ideally I would have had issued_at as part of the hash, since it should be read from the server. But for now I decided to calculate it (calculated_issued_at), I think it might be good enough.

[coveralls]

Pull Request Test Coverage Report for Build 637

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at ?%

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Totals
Change from base Build 625:	0.0%
Covered Lines:
Relevant Lines:	0

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💛 - Coveralls

[klippx]

Closing this as the backwards calculation doesn't even work, we use Time.now + expires_in - expires_in which gives Time.now again, which means clock skew is always zero.

Real fix in #391