Fix issue with attr_protected where malformed input could circumvent protection

Fixes: CVE-2013-0276

Conflicts:
	activemodel/lib/active_model/attribute_methods.rb
	activerecord/test/cases/mass_assignment_security_test.rb

Signed-off-by: Chris Houhoulis <chrish@interlink1.com>
fix serialization vulnerability

Signed-off-by: Chris Houhoulis <chrish@interlink1.com>