Instance level writers can have an impact on how the Active Model /
Record objects are saved.  Specifically, they can be used to bypass
validations.  This is a problem if mass assignment protection is
disabled and specific attributes are passed to the constructor.

Conflicts:
	activerecord/lib/active_record/scoping/default.rb
	activesupport/lib/active_support/callbacks.rb

CVE-2016-0753