Fix session check crash due to odoo recent change [1]

Odoo introduced a token on 2018-03-19, this token is
used for checking the rights.

The token itself is generated from data as a hash.
The redis_session module handle it correctly by saving it aside other
data in json format. Nevertheless, when reading it, from json format,
it forces all strings to unicode whereas some dumped string where str.

Thus it fails on comparison between str and unicode.

This fix solves it by always using token in unicode format.

[1] https://github.com/odoo/odoo/pull/22612

session_redis/models/__init__.py

@@ -0,0 +1 @@
+from . import user