cut-off timestamp after second part (10th digit)

[KOREAN139]

as time.Now().Unix() returns timestamp with 10 digits,
which records to second unit, we need to cut smaller than
second unit (milisecond, nanosecond) to compare timestamps
with integer comparison.

before this PR, integer comparison with iat, exp, nbf and now
can make a correct result under assumption that iat, exp, nbf
is 10-digits timestamp (which smallest unit is second).
if iat, exp, nbf has more digits, now is always smaller than
these timestamps, and so always make incorrect verify results.

so i added simple function func cutOff which cuts timestamp
to have first 10-digits only, to satisfy that assumption.