Reset ActionController::Base.allow_forgery_protection in controller specs

The controller_example_group adds a before block to tests that sets the class variable ActionController::Base.allow_forgery_protection to false, but never resets it.  This means that if a controller spec is run before any other spec that expects csrf protection to be enabled the second test will fail.  This commit adds an after block that sets ActionController::Base.allow_forgery_protection class variable back to what it was before.

There is not a test case added for this because we could not find any tests for the ActionController::Base.allow_forgery_protection setting in the first place.  If you can suggest the correct way to test this we will gladly add it.