fix(dav): return 405 when collection already exists

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

SoObjects/Appointments/SOGoAppointmentFolder.m

@@ -3076,15 +3076,15 @@ firstInstanceCalendarDateRange: (NGCalendarDateRange *) fir
 }
 
 /* folder management */
-- (BOOL) create
+- (NSException *) create
 {
-  BOOL rc;
   SOGoUserSettings *userSettings;
   NSMutableDictionary *calendarSettings;
+  NSException *error;
   SOGoUser *ownerUser;
 
-  rc = [super create];
-  if (rc)
+  error = [super create];
+  if (!error)
     {
       ownerUser = [SOGoUser userWithLogin: [self ownerInContext: context]];
       userSettings = [ownerUser userSettings];
@@ -3097,7 +3097,7 @@ firstInstanceCalendarDateRange: (NGCalendarDateRange *) fir
       [userSettings synchronize];
     }
 
-  return rc;
+  return error;
 }
 
 - (void) removeFolderSettings: (NSMutableDictionary *) moduleSettings

SoObjects/Appointments/SOGoAppointmentFolders.m

@@ -398,20 +398,18 @@ static SoSecurityManager *sm = nil;
   if ([subfolderNames containsObject: newName])
     {
       content = [NSString stringWithFormat: @"A collection named '%@' already exists.", newName];
-      error = [NSException exceptionWithDAVStatus: 403
+      error = [NSException exceptionWithDAVStatus: 405
                                            reason: content];
     }
   else
     {
-      properties = [[createContext request]
-		     davPatchedPropertiesWithTopTag: @"mkcalendar"];
+      properties = [[createContext request] davPatchedPropertiesWithTopTag: @"mkcalendar"];
       setProperties = [properties objectForKey: @"set"];
       newDisplayName = [self _fetchPropertyWithName: @"{DAV:}displayname"
                                             inArray: setProperties];
       if (![newDisplayName length])
 	newDisplayName = newName;
-      error
-	= [self newFolderWithName: newDisplayName andNameInContainer: newName];
+      error = [self newFolderWithName: newDisplayName andNameInContainer: newName];
       if (!error)
 	{
 	  newFolder = [self lookupName: newName