initialize temporal classes of subclasses (#39)

* initialize temporal classes of subclasses

history classes where not correctly initialized in such a case:

```rb
class User
end

class BigUser < User
end

User.history.last # => exception if it is a BigUserHistory without this patch
```

* rubocop

lib/temporal_tables/temporal_class.rb

@@ -59,6 +59,9 @@ def find_sti_class(type_name)
         type_name += 'History' unless type_name =~ /History\Z/
 
         begin
+          # Calling .history makes sure history class is created
+          type_name.sub(/History$/, '').constantize.history
+
           super
         rescue ActiveRecord::SubclassNotFound
           superclass.send(:find_sti_class, type_name)

spec/basic_history_spec.rb

@@ -156,6 +156,13 @@
       end
     end
 
+    describe 'when working with STI and using superclass history' do
+      it 'allows to fetch history entry of subclass history' do
+        Broom.create person: emily, model: 'Cackler 2000'
+        expect(FlyingMachine.history.last.class.name).to eql('BroomHistory')
+      end
+    end
+
     describe 'when working with STI one level deep' do
       let!(:broom) { Broom.create person: emily, model: 'Cackler 2000' }