Minor change to JSON parsing for FBJSAjax + JSON

Our JSON parsing engine will change slightly on the 20th of January. This affects you ONLY if you use FBJSAjax with the JSON responseType, and your application emits JSON which does not conform to the JSON specification but which we currently parse anyway. (Even in this case, it is unlikely you are affected.)

More detail (including a test console) is available in this post on the developer forum.