Tus endpoints may return malformed expiry header

[butonic]

The pkg/rhttp/datatx/manager/tus/tus.go setHeaders() function returns the expires metadata as the Tus Upload-Expires header:

	expires := info.MetaData["expires"]
	if expires != "" {
		w.Header().Set(net.HeaderTusUploadExpires, expires)
	}

The storageprovider.go sets expires as a unix timestamp in InitiateFileUpload():

	var expirationTimestamp *typesv1beta1.Timestamp
	if s.conf.UploadExpiration > 0 {
		expirationTimestamp = &typesv1beta1.Timestamp{
			Seconds: uint64(time.Now().UTC().Add(time.Duration(s.conf.UploadExpiration) * time.Second).Unix()),
		}
		metadata["expires"] = strconv.Itoa(int(expirationTimestamp.Seconds))
	}

This violates the Tus spec: Upload-Expires MUST be RFC 7231 datetime format, see https://tus.io/protocols/resumable-upload#upload-expires

We need to parste the timestamp and convert it. Aaaaand we should not use the tus metadata to transport the expiry. That is just a hacky workaround, anyway.