Python3: fix test_insecure

test_insecure() fails because it ends up calling guess_json_utf8() in the
requests module, where the count() method of the str type is called with bytes
input:

    nullcount = sample.count(_null)

This patch makes sure that we pass 'sample' as bytes.

Change-Id: I70071623d425ebbf910d3568a7b5c5d857892402
Merge "Python3: fix test_insecure"