use strict datetime value in schema

tracim/tests/functional/test_session.py

 # coding=utf-8
+import datetime
 import pytest
 from sqlalchemy.exc import OperationalError
 
             params=params,
             status=200,
         )
+        assert res.json_body['created']
+        datetime.datetime.strptime(
+            res.json_body['created'],
+            '%Y-%m-%dT%H:%M:%SZ'
+        )
         assert res.json_body['public_name'] == 'Global manager'
         assert res.json_body['email'] == 'admin@admin.admin'
-        assert res.json_body['created']
         assert res.json_body['is_active']
         assert res.json_body['profile']
         assert res.json_body['profile'] == 'administrators'

tracim/views/core_api/schemas.py

         example='Suri Cate',
     )
     created = marshmallow.fields.DateTime(
-        format='iso8601',
-        description='User account creation date (iso8601 format).',
+        format='%Y-%m-%dT%H:%M:%SZ',
+        description='User account creation date',
     )
     is_active = marshmallow.fields.Bool(
         example=True,