Make associations respect store_full_sti_class

Currently when doing an association preload, it will respect store_full_sti_class and use sti_name for the class it is loading.

However, all of the association code that actually sets the value of these columns uses base_class.name rather than sti_name.

This means association preloading won't work on polymorphic associations if you have store_full_sti_class disabled.

This patch creates a polymorphic_name class method that just calls base_class.sti_name and then uses that in both the association preload code, as well as when the _type column gets set in the association code.

Test is included.