toXML method
Implementation
@override
String toXML() {
return '''<?xml version="1.0" encoding="utf-8"?>
<cupps xmlns="$xmlns" messageID="$messageID" xmlns:xsi="$xsi" messageName="$messageName">
<$messageName deviceName="$deviceName" deviceToken="$deviceToken" airlineID="$airlineID"/>
</cupps>''';
}