Rails 4.1.0.beta1 Fix

In Edge Rails, `ActionController::Base.protected_instance_variables` is a Set rather than an Array (see https://github.com/rails/rails/commit/c8b566d54da278a8e675115bcf2e9590f75f5eb5), and so calling `#push` raises a NoMethodError. This tiny change uses the `#+` operator instead, which behaves as expected for Sets and Arrays.