Eliminate instance level writers for class accessors

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

activemodel/lib/active_model/serializers/json.rb

@@ -10,7 +10,7 @@ module ActiveModel
       included do
         extend ActiveModel::Naming
 
-        class_attribute :include_root_in_json
+        class_attribute :include_root_in_json, instance_writer: false
         self.include_root_in_json = false
       end
 

activemodel/lib/active_model/validations.rb

@@ -46,9 +46,10 @@ module ActiveModel
       include HelperMethods
 
       attr_accessor :validation_context
+      private :validation_context=
       define_callbacks :validate, scope: :name
 
-      class_attribute :_validators
+      class_attribute :_validators, instance_writer: false
       self._validators = Hash.new { |h,k| h[k] = [] }
     end
 

activerecord/lib/active_record/enum.rb

@@ -68,7 +68,7 @@ module ActiveRecord
   # Where conditions on an enum attribute must use the ordinal value of an enum.
   module Enum
     def self.extended(base) # :nodoc:
-      base.class_attribute(:defined_enums)
+      base.class_attribute(:defined_enums, instance_writer: false)
       base.defined_enums = {}
     end
 

activerecord/lib/active_record/reflection.rb

@@ -4,8 +4,8 @@ module ActiveRecord
     extend ActiveSupport::Concern
 
     included do
-      class_attribute :_reflections
-      class_attribute :aggregate_reflections
+      class_attribute :_reflections, instance_writer: false
+      class_attribute :aggregate_reflections, instance_writer: false
       self._reflections = {}
       self.aggregate_reflections = {}
     end

activesupport/lib/active_support/callbacks.rb

@@ -768,7 +768,7 @@ module ActiveSupport
         end
 
         names.each do |name|
-          class_attribute "_#{name}_callbacks"
+          class_attribute "_#{name}_callbacks", instance_writer: false
           set_callbacks name, CallbackChain.new(name, options)
         end
       end