initialize temporal classes of subclasses

[doits]

history classes where not correctly initialized in such a case:

class User
end

class BigUser < User
end

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

[doits]

While working on a spec I realized that my first attempt fixed the error only if the subclass was already loaded, which it might not be the case when initializing the history class of the superclass, e.g.:

# without eager loading

FlyingMachine.subclasses # => []
# FlyingMachine.history would not init BroomHistory in this case with my old fix

Broom
# subclass is loaded now

FlyingMachine.subclasses # => [Broom]
# FlyingMachine.history would init BroomHistory if it was called after this point *for the first time*

Therefore I changed the fix to work differently and now it always works (and I added a spec).

What do you think about it?

[bkroeker]

Thank you!