Force use_strict during testing

[lionello]

Our project was using JS strict mode and we got errors like

TypeError: Cannot create property 'iat' on string

Enabling --use_strict during the mocha test revealed the culprit.

[ziluvatar]

Could you add some tests on claim-iat.test.js file to assert that behavior is not changing for string payloads (proper stringified json and just a plain string like in your issue)

[lionello]

@ziluvatar Done. Please check if it's what you had in mind.

[ziluvatar]

Thanks @lionello 👏