Fix the issue of renaming user categories to the same name it already has resulting in deletion of the category (see https://bugs.launchpad.net/calibre/+bug/1207131)
Merge branch 'master' of https://github.com/brandizzi/calibre

Fix renaming a user category to the same name it already has erases the
user category. Fixes #1207131 [Renaming a user category to the same name it already has erase this same user category](https://bugs.launchpad.net/calibre/+bug/1207131)