Fix parameter filtering in Rails 2.

Rails 2 does parameter filtering via a controller configuration method that [dynamically defines a method](https://github.com/rails/rails/blob/2-3-stable/actionpack/lib/action_controller/base.rb#L486) on the controller, so the configured parameters aren't easily accessible. Intercept these parameters as they're configured so that the Bugsnag configuration can take them into account.

I ran into this issue because I have an old Rails 2 project with custom filtered parameters and I noticed they were being sent to Bugsnag without being filtered out.

I originally opened #171 to fix this issue but then realized that that fix can be made much simpler. This is the simplified fix.