Add calendar URI before requesting calendar object.

If the custom CalDAV backend is called via this stack trace:

```
 4. Sabre\DAV\Server->invokeMethod() /home/jan/horde-git/framework/Dav/vendor/sabre/dav/lib/Sabre/DAV/Server.php:214
 5. Sabre\DAV\Server->broadcastEvent() /home/jan/horde-git/framework/Dav/vendor/sabre/dav/lib/Sabre/DAV/Server.php:455
 6. call_user_func_array() /home/jan/horde-git/framework/Dav/vendor/sabre/dav/lib/Sabre/DAV/Server.php:433
 7. Sabre\DAV\Browser\Plugin->httpGetInterceptor()
 8. Sabre\DAV\Browser\Plugin->generateDirectoryIndex() /home/jan/horde-git/framework/Dav/vendor/sabre/dav/lib/Sabre/DAV/Browser/Plugin.php:137
 9. Sabre\DAV\ObjectTree->getNodeForPath() /home/jan/horde-git/framework/Dav/vendor/sabre/dav/lib/Sabre/DAV/Browser/Plugin.php:343
10. Sabre\CalDAV\Calendar->getChild() /home/jan/horde-git/framework/Dav/vendor/sabre/dav/lib/Sabre/DAV/ObjectTree.php:72
11. Horde_Dav_CalDav->getCalendarObject() /home/jan/horde-git/framework/Dav/vendor/sabre/dav/lib/Sabre/CalDAV/Calendar.php:110
```

the parameter $objectUri to getCalendarObject() was an incomplete URI that only contains the object's base name.