more specific exception in ActiveResource

Hi all, I created a patch wich allows to a nested resource raise an ActiveResource::MissingPrefixParam exception instead of ActiveResource::ResourceNotFound when we call a method without prefix_params, IMO prefix_params should be required on nested resource methods that uses element_path or collection_path. The ticket with the patch and tests:

https://rails.lighthouseapp.com/projects/8994/tickets/5631-activeresourcemissingprefixparam-proposal

Any comments and feedback would be awesome, If you're interested, please test and +1.