Use simpler os.Create for writing files

storage/filesystem.go

@@ -298,7 +298,7 @@ func (b *filesystemBackend) PutAddressObject(ctx context.Context, path string, c
 		return "", carddav.NewPreconditionError(carddav.PreconditionNoUIDConflict)
 	}
 
-	f, err := os.OpenFile(localPath, os.O_RDWR|os.O_CREATE, 0644)
+	f, err := os.Create(localPath)
 	if err != nil {
 		return "", err
 	}