Python3: fix a bytes/str issue

test_curl_log_i18n_headers, introduced in
0b730e82b4bf63be1c3acd924a77a684a31db709, does not work with Python 3, because
Python 3, the headers must also be bytes,

Change-Id: I7baf737b3d9e2763a43eb593ca8ea733274a1107
Merge "Python3: fix a bytes/str issue"