public void HttpError()
 {
     // Arrange
     A.CallTo(_mockMessageHandler)
     .Where(x => x.Method.Name == "SendAsync")
     .WithReturnType <Task <HttpResponseMessage> >()
     .Returns(Task.FromResult(new HttpResponseMessage
     {
         StatusCode = HttpStatusCode.InternalServerError,
         Content    = new StringContent("abcd")
     }));
     // Act
     var result = _systemUnderTest.DoSomething();
     // Assert
     // Assert.AreEqual(...);
 }