Fix ActiveRecord `subclass_from_attrs` when eager_load is false.

It cannot find subclass because all classes are loaded automatically
when it needs. So this way for development mode if we have
```
class Person < AR
end

class User < Person
end

Person.subclasses #=> will be []
```
And `Person.new(type: 'User')` will raise error `ActiveRecord::SubclassNotFound`