Fix logic error

- Should be a not on line 356 of event.js

lib/event.js

@@ -353,7 +353,7 @@ var ICalEvent = function(_data, calendar) {
 
             data.repeating.exclude = [];
             repeating.exclude.forEach(function(excludedDate) {
-                if(Object.prototype.toString.call(excludedDate) === '[object Date]') {
+                if(!(Object.prototype.toString.call(excludedDate) === '[object Date]')) {
                     throw '`repeating.exclude` contains invalid value `' + excludedDate + '`!';
                 }