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.
Better fix with @rwz's suggestion
Fix 'wrong constant name' error (Rails 4.1.0beta1)