Book polishing: Fix publisher field in epub not being cleared when updating metadata. Fixes #1319732 [calibre will not update Publisher](https://bugs.launchpad.net/calibre/+bug/1319732)
Fix null fields not being cleared when updating metadata before edit book.
Kobo driver: Update to handle changes to annotation processing in newer Kobo firmware.

The changed contentIds for kepubs meant not all annotations were being
retrieved. This also meant that some annotations would be out of order.
Fixes #1319819 [Annotations for kepubs now use different contentIds](https://bugs.launchpad.net/calibre/+bug/1319819)