Commented Issue: [Required] should not present users with duplicate errors [590]

See: http://aspnetwebstack.codeplex.com/discussions/396199

The implementation change here is to not recognize the [Required] model validator for reference types when doing the value validation.
Comments: Checked in: https://aspnetwebstack.codeplex.com/SourceControl/changeset/353911090f0c02497d4003cd35a0effce43f0adf