Mass assignment with inheritance column

Related to rails commit:
https://github.com/rails/rails/commit/89b5b31cc4f8407f648a2447665ef23f9024e8a5

which adds STI support to init and building associations, allowing
you to do

* BaseClass.new(:type => "SubClass")
* parent.children.build(:type => "SubClass")
* parent.build_child(:type => "SubClass")

to initialize an STI subclass.

This commit ensures that a protected inheritance column is still
respected during init