Update Sabre to version 1.7.1

3rdparty/Sabre.includes.php

3rdparty/Sabre/CalDAV/Backend/Abstract.php

3rdparty/Sabre/CalDAV/Backend/BackendInterface.php

3rdparty/Sabre/CalDAV/Backend/NotificationSupport.php

3rdparty/Sabre/CalDAV/Backend/PDO.php

3rdparty/Sabre/CalDAV/Backend/SharingSupport.php

3rdparty/Sabre/CalDAV/Calendar.php

3rdparty/Sabre/CalDAV/CalendarObject.php

3rdparty/Sabre/CalDAV/CalendarQueryParser.php

3rdparty/Sabre/CalDAV/CalendarQueryValidator.php

3rdparty/Sabre/CalDAV/CalendarRootNode.php

3rdparty/Sabre/CalDAV/Exception/InvalidComponentType.php

3rdparty/Sabre/CalDAV/ICSExportPlugin.php

3rdparty/Sabre/CalDAV/ICalendar.php

3rdparty/Sabre/CalDAV/ICalendarObject.php

3rdparty/Sabre/CalDAV/IShareableCalendar.php

3rdparty/Sabre/CalDAV/ISharedCalendar.php

3rdparty/Sabre/CalDAV/Notifications/Collection.php

3rdparty/Sabre/CalDAV/Notifications/ICollection.php

3rdparty/Sabre/CalDAV/Notifications/INode.php