Cookie chapter fixes

Apart from the two tiny fixes here, two suggestions in comments at:
1. https://github.com/leifos/tango_with_django_book/commit/d7dd916e9f94cac4cd5843cb0d9a6c2315b735d6#commitcomment-4803526
2. https://github.com/leifos/tango_with_django_book/commit/f1134bd2329937b8b3cce69e034a6d24929b42fa#commitcomment-4803750