Fixed 'exp' handling in ID token

[sungjungk]

It is a measure for issue #57.

Modification:
Change the way of ‘exp’ handling; from data.GetString(“exp”, &str_value) to data.GetScalar(“exp”, &int_value)

Result:
It will not happen the bug when receiving ID token value.

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[sungjungk]

I signed it!

[googlebot]

CLAs look good, thanks!

[sungjungk]

@SurferJeffAtGoogle Could you check the issue?

[SurferJeffAtGoogle]

Thank you for submitting this fix. I apologize for taking so long to review. It got lost my TODO list. 😞