public async Task TestShouldGetInvoices()
        {
            // get invoices between two dates
            var invoices = await _bitpay.GetInvoices(yesterday, tomorrow);

            Assert.True(invoices.Count > 0, "No invoices retrieved");
        }
        public async Task TestShouldGetInvoices()
        {
            // get invoices between two dates
            var invoices = await _bitpay.GetInvoices(new DateTime(2018, 8, 1), new DateTime(2018, 11, 30));

            Assert.IsTrue(invoices.Count > 0, "No invoices retrieved");
        }