moodle: how to change webservice message from xml to json

I want to use json format for moodle webservices I have configured REST protocol in Site administration / ► Plugins / ► Web services / ► Manage protocols but when I try this call from browser http://localhost/moodle/webservice/rest/server.php?wsfunction=core_enrol_get_users_courses&wstoken=a6e17971d60ddcd8f7ea648a605cebb8&userid=4 the answer is in XML format and not in JSON format. What I need to do to set up JSON format? thank you