Fix sogo_folder_get_deleted_fmids() to return 0 as a globcnt (leads to crashes in OC layer)

The important commit (among bogus ones) is this 3edb5fc4bd542571e9848c8c2dce67bfb4d6f8fc
With this commit, SOGo layer is able to correctly remove records for deleted messages
and store "SyncLastDeleteChangeNumber" for parent folder.

it wasn't working in first place due to the fact, that SOGo wasn't able to update its cache
because it was trying in to search in cached Dictionary with a key of type NSNumber while
dictionary keys are of type NSString.
So it boils down to comparisons like [NSString isEqual:NSNumber] -> always FALSE (NO)