fix(dav): return 405 when collection already exists

See https://datatracker.ietf.org/doc/html/rfc4918#section-9.3.1

SoObjects/SOGo/SOGoGCSFolder.h

@@ -1,6 +1,6 @@
 /*
   Copyright (C) 2004-2005 SKYRIX Software AG
-  Copyright (C) 2006-2015 Inverse inc.
+  Copyright (C) 2006-2022 Inverse inc.
 
   This file is part of SOGo.
 
@@ -96,7 +96,7 @@
 
 - (BOOL) folderIsMandatory;
 
-- (BOOL) create;
+- (NSException *) create;
 - (NSException *) delete;
 - (void) renameTo: (NSString *) newName;