Provide session defaults.

Provide sane defaults to make testing easier.
Cake3 plugins would throw errors currently when trying to run phpunit testing:

    ini_set(): session.name cannot be a numeric or empty ''

see [here](https://travis-ci.org/dereuromark/cakephp-tools/jobs/31455522) for example.

This is easily solvable by not requireing it in the bootstrap.
It should default to php on its own.