1. There are some problems about the test method.
problem 1:
Unit tests may not cover our function, it depends on the python version
that performed the test.
problem 2:
when using function 'constant_time_compare(first, second)', 'first' and
'second' params are usually HMAC digest values, it is not appropriate to
use utf-8 encoded values as mock data.
2. The previous commit `f1d332a` lead into a bug, but due to the problem 1
and the problem 2, we did not find out the error.
Change-Id: I1c29bfe69f8eda60f3c5caaf3e5447dd5b69b108
Closes-Bug: #1772851
e158c10ccb