Revert "raise error for acts_as_paranoid on models without primary keys"

This reverts commit 6881e750aba90fe205fe3a91b4ed92067227dfd5.
test: Rename connect! to setup!; use ensure

Change connect! to setup!, so that we can use connect! to give us an
empty DB with no tables.

Use ensure where appropriate to ensure that tests after that one can run
even if it fails.
Test using acts_as_paranoid class without DB

Regression from 6881e75

We need to be able to load our class definitions without having an
available database connection.
Simplify default_sentinel_value