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 !