title: EventSources with sql not working in Fullcalendar
body: <p>Hello im trying to use EventSources to get multiple events in the same calendar with sql, but when I try to do it like the fullcalendar docs the calendar goes blank. What am I doing wrong? Here is my code for the callendar</p>
  
  <pre><code>                      &lt;script type="text/javascript"&gt;
                      document.addEventListener('DOMContentLoaded', function() { // DOMContentLoaded zorgt ervoor dat eerst de html laad
                        var calendarEl = document.getElementById('calendar'); // grab element reference
  
                        var calendar = new FullCalendar.Calendar(calendarEl, {
                          // Opties in fullcalendar
                          eventLimit: true,
                          locale: 'nl',
                          buttonText: {
                            month: 'Maand',
                            agendaDay: 'Dag',
                            agendaWeek: 'Week',
                            today: 'Vandaag',
                            listMonth: 'Lijst'
                          },
                          header: {
                            left: 'prev,next today',
                            center: 'title',
                            right: 'listMonth,month,agendaWeek,agendaDay'
                          },
                          eventSources: [
                            '/sql.php',
                            '/availability.php'
                          ]
                        });
  
                        calendar.render();
                      });
                    &lt;/script&gt;
  </code></pre>