PATCH ActiveResource: support validations errors messages in json

Hi,

i submitted a patch a week ago to support validation messages in json
for ActiveResource. Currently, only xml are supported and the xml
parser is called even if response is in json format.

Ticket is at http://rails.lighthouseapp.com/projects/8994/tickets/1956-json-validations-errors-for-activeresource

and patch itself :
http://rails.lighthouseapp.com/attachments/88393/json_validations_for_active_resource.diff

Thanks.