GSBlog->saveCategory doesn't check if the category already exists

[John Stray]

In the primary class, the method ->saveCategory() does not check if the category already exists before trying to save it. This results in duplicate categories being created, and therefor incorrect post category attribution.

Function will be modified so that it checks for the existence of the given category before attempting to update the list.

Sidenote: The function argument $existing=false is never used in the code. This will be removed because the code will check anyway, plus a new function exists for updating categories.