iCalCalendar clear() fix

lib/calendar.js

@@ -435,6 +435,8 @@ var ICalCalendar = function(_data) {
         data.prodid = '//sebbo.net//ical-generator//EN';
         data.name = null;
         data.timezone = null;
+        data.ttl = null;
+        data.url = null;
 
         data.events = [];
         return this;