Do not change json objects created with render :json <object> calls

Hello Muriel-Salvan,

I noticed ( a bit late due to a lack of continuous integration ), that when requesting json representations of our data the response was still in the for of { "div#main": <json-representation> ...}. It would have led to a bit of refactoring, so I changed the behavior of rails-ajax to simply do apply default rails-behaviour when render :json is called.

Hopefully you agree with this change. Of course a boolean to force default behavior could also be introduced, but I have no use for it now. Maybe somebody else will come along with that request.

Kind regards,
LoveIsGrief