STI test failure when looking up record from SubClass

```ruby
class Journalist < ActiveRecord::Base
  extend FriendlyId
  friendly_id :name, :use => :slugged
end

class Editorialist < Journalist
  extend FriendlyId
  friendly_id :name, :use => :slugged
end

editorialist = Editorialist.create name: "foo"

Editorialist.find editorialist.slug
# => Couldn't find Editorialist 
```

Also:
```ruby
class Journalist < ActiveRecord::Base
end

class Editorialist < Journalist
  extend FriendlyId
  friendly_id :name, :use => :slugged
end

Editorialist.create name: "foo"
# => Couldn't create Editorialist 
`undefined method `exists_by_friendly_id?'`
```