STI debug

Hi

I dont know if it's a good way to fix that, but solialization gem doesn't work well with STI only because rails uses ```base_class```to store the class name in polymorphic column.

https://github.com/rails/rails/blob/master/activerecord/lib/active_record/associations/belongs_to_polymorphic_association.rb#L14

I could add test if you think it worths to merge it 

Thanks a lot for your gem!
Seb