public async void ChangeAuthenticationHeader_ShouldChangeAuthenticationHeader()
        {
            // Arrange
            const string testHeader = "TestHeader";

            _mockHttpMessageHandler
            .Protected()
            .Setup <Task <HttpResponseMessage> >(
                "SendAsync",
                ItExpr.IsAny <HttpRequestMessage>(),
                ItExpr.IsAny <CancellationToken>()
                )
            .ReturnsAsync(new HttpResponseMessage
            {
                StatusCode = HttpStatusCode.OK
            })
            .Verifiable();

            // Act
            _gateway.ChangeAuthenticationHeader(testHeader);
            await _gateway.GetAsync(It.IsAny <string>());

            // Assert
            _mockHttpMessageHandler
            .Protected()
            .Verify(
                "SendAsync",
                Times.Once(),
                ItExpr.Is <HttpRequestMessage>(req =>
                                               req.Headers.Authorization.ToString() == testHeader),
                ItExpr.IsAny <CancellationToken>()
                );
        }