public async Task StubIntegration_RegularGet_Dynamic_UtcNow()
        {
            // arrange
            var          url = $"{TestServer.BaseAddress}dynamic-utc-now.txt";
            const string expectedDateTime = "2019-08-21 20:41:51";
            var          expectedResult   = $"UTC now: {expectedDateTime}";

            var now = new DateTime(2019, 8, 21, 20, 41, 51, DateTimeKind.Utc);

            DateTimeMock
            .Setup(m => m.UtcNow)
            .Returns(now);

            var request = new HttpRequestMessage(HttpMethod.Get, url);

            // act / assert
            using var response = await Client.SendAsync(request);

            var content = await response.Content.ReadAsStringAsync();

            Assert.AreEqual(expectedResult, content);
            Assert.AreEqual(HttpStatusCode.OK, response.StatusCode);
            Assert.AreEqual("text/plain", response.Content.Headers.ContentType.ToString());

            Assert.AreEqual(expectedDateTime, response.Headers.Single(h => h.Key == "X-Header").Value.Single());
        }