If thumbnail is being updated, use object from islandora_object_load() instead

Attempting to change/re-upload an entity thumbnail from 7.x HEAD results in an error:

```RepositoryException: Conflict in RepositoryConnection->parseFedoraExceptions() (line 229 of /var/www/drupal/sites/all/libraries/tuque/RepositoryConnection.php).
```

Fedora error is:

https://gist.github.com/JacobSanford/72a401e5bbd8f334a9e4

This seemed very similar to that mentioned:
https://github.com/ruebot/islandora_checksum/issues/5
and solved:
https://github.com/Islandora/islandora_xml_forms/commit/40860180622b6e0f474604498c94f7f952b6e0c4

I modeled the fix after that.