Avoid hardcoded `request_forgery_protection_token` value in teardown.

Refer to https://github.com/rails/rails/blob/master/actionpack/lib/action_controller/metal/request_forgery_protection.rb#L60-L61.

`request_forgery_protection_token` is set to `:authenticity_token` by default, instead of `nil`.

\cc @senny