STI test failure

adding finders to both
cleaned up test

sub class of STI can't create records if parent
does not extend friendly id
Ensure finder methods are included in STI classes

Without this change, STI classes skipped adding the finders because
FriendyIdConfig#uses? would return true for FriendlyId::Finders when the
parent class had this module loaded, so the module loading would always
be skipped in the child module.

However, since FriendId::Finders.setup is always invoked when adding a
FriendlyId addon, the module will now get added to child classes.

See #516
Fix slug creation in STI classes

Previously, if the child class had the finder modules loaded but the
parent class did not, then slugs could not be created because the slug
generator assumed friendly finders were available, but were not because
the scope is uses comes from the super class rather than the child
class.

The child class scope is not used in order to maintain slug uniqueness
across child classes.

See #516
Extend FriendlyId in STI superclass.
Spelling fix