Update davclient.js for headers fix

core/vendor/davclient.js/lib/client.js

@@ -164,7 +164,8 @@ dav.Client.prototype = {
 
         var self = this;
         var xhr = this.xhrProvider();
-
+        headers = headers || {};
+        
         if (this.userName) {
             headers['Authorization'] = 'Basic ' + btoa(this.userName + ':' + this.password);
             // xhr.open(method, this.resolveUrl(url), true, this.userName, this.password);