rails/rails — pull request #5049 by fabioyamate (2012-02-15 13:17:50+00:00)
Title: Fix sanitize_for_mass_assigment when role is nil

There is an example in Rails documentation that suggests implementing
assign_attributes method for ActiveModel interface, that by default
sends option role with nil. Since mass_assignment_authorizer never
is called without args, we can move the default value internally.

This should be backported to Rails 3.2