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
```

lib/temporal_tables/history_hook.rb

@@ -35,6 +35,11 @@ def self.history # rubocop:disable Metrics/MethodLength, Metrics/AbcSize
           # Traverse associations and make sure they have
           # history classes too.
           history_class.constantize.temporalize_associations!
+
+          # Traverse subclasses and make sure they have
+          # history classes too.
+          history_class.constantize.temporalize_subclasses!
+
           history_class.constantize
         end
       end

lib/temporal_tables/temporal_class.rb

@@ -47,6 +47,18 @@ def self.temporalize_associations! # rubocop:disable Metrics/MethodLength, Metri
             )
           end
         end
+
+        # Iterates all subclasses, makes sure their history classes are
+        # created and initialized
+        def self.temporalize_subclasses!
+          orig_class.subclasses.each do |subclass|
+            # skip temporal classes: they are subclasses, too
+            next if subclass.include?(TemporalClass)
+
+            # call .history to ensure temporal class is created
+            subclass.history
+          end
+        end
       end
     end