Fix to allow ActiveRecord tests to be run individually
Fix to allow ActiveRecord tests to be run individually

* i.e. `ruby test/active_record_adapter/slug_test.rb`.
* ActiveSupport aliases the constant MissingSourceError to LoadError,
  but in this case, the AS core_ext has not yet been loaded and a
  the constant `MissingSourceError`is not resolved.

Signed-off-by: Alex Coles <60c6d277a8bd81de7fdde19201bf9c58a3df08f4@alexcolesportfolio.com>

Pathgather/friendly_id,ThanhKhoaIT/friendly_id,norman/friendly_id,zenhacks/friendly_id,joshsoftware/friendly_id,kangkyu/friendly_id,tekin/friendly_id,sideci-sample/sideci-sample-friendly_id,morsedigital/friendly_id,MatthewRDodds/friendly_id,MAubreyK/friendly_id,ashagnanasekar/friendly_id,Empact/friendly_id,onursarikaya/friendly_id