Just came up across this very useful post : http://sadomovalex.blogspot.ca/2015/08/fix-problem-with-returned-xml-instead.html
It shows how to use JSON light with SP2013 SP1+.

Just came up across this very useful post : http://sadomovalex.blogspot.ca/2015/08/fix-problem-with-returned-xml-instead.html
It shows how to use JSON light with SP2013 SP1+.