/// <summary>
        /// Actualiza una factura
        /// </summary>
        /// <param name="customerId">Identificador del cliente</param>
        /// <param name="contractId">Identificador del contrato</param>
        /// <param name="invoiceNumber">Número de factura</param>
        /// <param name="invoice">Factura</param>
        /// <returns></returns>
        public Task<Invoice> UpdateInvoiceAsync(string customerId, string contractId, string invoiceNumber, Invoice invoice)
        {
            string url = GetUrl(_invoiceItemResourceUrl, customerId, contractId, invoiceNumber);

            return PutAsync<Invoice, Invoice>(url, invoice);
        }
        /// <summary>
        /// Añade un nuevo contrato a un cliente
        /// </summary>
        /// <param name="customerId">Identificador del cliente</param>
        /// <param name="contractId">Identificador del contrato</param>
        /// <param name="invoice">Factura</param>
        /// <returns></returns>
        public Task<Invoice> AddInvoiceAsync(string customerId, string contractId, Invoice invoice)
        {
            string url = GetUrl(_invoiceCollectionResourceUrl, customerId, contractId);

            return PostAsync<Invoice, Invoice>(url, invoice);
        }