patch to support activeresource validations in json

Hi,

i wrote this patch a few weeks ago and updated it to latest rails version available on github.

If anyone can take a look and leave me a few comments about it, it would be great.

Currently, ActiveResource does supports validations (like ones in ActiveRecord) but only in XML format. This patch add json support for validations and comes with unit tests :slight_smile:

https://rails.lighthouseapp.com/projects/8994-ruby-on-rails/tickets/1956-json-validations-errors-for-activeresource

Thanks !