Awaited call loses context in Unit Test

Question

We have the following ASP.NET code (I've cut it down for clarity):

var httpContent = new StringContent(postData, Encoding.UTF8);
using (var client = this.GetClient(url, contentType))
{
    using (var response = await client.PostAsync(url, httpContent).ConfigureAwait(true);
    {
        ret.Response = await response.Content.ReadAsStringAsync().ConfigureAwait(true);
    }
}

'client' is an HttpClient object when run in PROD, but in our Unit Test it's:

MockRepository.GenerateMock<IHttpClient>();

Notice that we use .ConfigureAwait(true). The reason for this is that we store objects in the HttpContext.Current.Items collection (this probably isn't ideal, but it is what it is). If we didn't persist the Thread's Context then HttpContext.Current returns as null.

This all works as expected when run in PROD.

However, when running the Unit Test, execution of the following line loses Context and HttpContext.Current becomes null:

await response.Content.ReadAsStringAsync().ConfigureAwait(true);

Note - the proceeding call of PostAsync on the Stub maintains the Context.

So how do we program the Stub?

using (var httpResponse = new HttpResponseMessage())
{
    httpResponse.Content = new StringContent(msg, Encoding.UTF8);
    httpResponse.StatusCode = HttpStatusCode.OK;

    this.FakeHttpClient
        .Stub(i => i.PostAsync("", "")).IgnoreArguments()
        .Return(Task.FromResult(this.httpResponse)).Repeat.Once();

    // Act
}

When the Test runs, the data is returned so the fake objects behave perfectly EXCEPT that the ReadAsStringAsync().ConfigureAwait(true) destroys the Context.

StringContent can't be mocked directly as it doesn't have a parameterless constructor. I created a wrapper class to see if I could mock that, but when I had the following code:

var fakeStringContent = MockRepository.GenerateStub<Wrap>();
fakeStringContent
    .Stub(fsc => fsc.ReadAsStringAsync())
    .Return(Task.FromResult("<response/>"))
    .Repeat.Once();

Then there was an Exception thrown on the .Stub(...) line which was:

System.InvalidOperationException: 'The async operation did not return a System.Threading.Tasks.Task object.'