In Python 3, one cannot write:
from tempfile import NamedTemporaryFile
with NamedTemporaryFile() as f:
f.write('foobar')
The input of f.write() must be bytes. Encode it when necessary in
middleware/auth_token.py, in a way that is compatible with Python 2.
Change-Id: Ib60afbc5e01c35f59cd7c9b68bfedb10ad897ff9