Redefine idempotentency to not also imply changes were made

Previously .idempotently would imply that a change had been made and that it
was idempotent.  This has changed to mean only idempotence, so a resource that
makes no change is now considered idempotent too.

To test a resource both makes a change and is idempotent, use
.with_change.idempotently instead of just .idempotently.

Fixes #7