serve webdav: fix --baseurl without leading /

The webdav server needs the prefix passed to it with a leading /
otherwise it does not remove it properly.

The docs state that a leading slash is optional so this patch adds one
if not present.

See: https://forum.rclone.org/t/cant-rename-files-in-rclone-serve-webdav-with-baseurl-maybe-wrong-handling-of-move-request-method/44637

cmd/serve/webdav/webdav.go

@@ -203,6 +203,9 @@ func newWebDAV(ctx context.Context, f fs.Fs, opt *Options) (w *WebDAV, err error
 		return nil, fmt.Errorf("failed to init server: %w", err)
 	}
 
+	// Make sure BaseURL starts with a / and doesn't end with one
+	w.opt.HTTP.BaseURL = "/" + strings.Trim(w.opt.HTTP.BaseURL, "/")
+
 	webdavHandler := &webdav.Handler{
 		Prefix:     w.opt.HTTP.BaseURL,
 		FileSystem: w,