Fix exception in in-memory permission backend (fixes #2687)

[cbguder]

Fix an exception in the in-memory permission backend when user principles exist, and get_accessible_objects is called without bound permissions.

Fixes #2687.

• Add documentation.
• Add tests.
• Add a changelog entry.
• Add your name in the contributors file.
• If you changed the HTTP API, update the API_VERSION constant and add an API changelog entry in the docs
• If you added a new configuration setting, update the kinto.tpl file with it.

[leplatrem]

Awesome, thank you very much!