Use static $timestamp instead of time()

[josephmcdermott]

I am unit testing an application where I want to assert that certain JWT values will in fact be marked as 'expired' or 'not ready' but it is proving difficult due to the dynamic values within JWT::decode() method generated by time().

This PR allows a static property to be specified, fixing the value of 'current time' for the test suite.

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[josephmcdermott]

I signed it!

[josephmcdermott]

I signed it! (again)

[josephmcdermott]

I signed it!

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CLAs look good, thanks!

[josephmcdermott]

The Google CLA approval was harder than the code change! :) FYI anybody else, I had to add a commit like this: git commit --author="MY_NAME <MY_GOOGLE_EMAIL_ADDRESS>" -m"..."