MVC4 / Mocking Controller.Request

I´m currently working on a MVC4 Project. As I did a few refactorings the tests should be changed too.

In the first scenario a session hash had to be passed included within the URL. That being nasted, I decided to pass it to the server as a Request header.

Example:

[HttpPost]
public ActionResult Generate(ClipGenerateRequest request)
{
    string hash = Request.Headers["hash"];

    ClipGenerationResponse responseModel = new ClipGenerationResponse();            

    return Json(responseModel, JsonRequestBehavior.AllowGet);
}

The problem now seems to be that I'm unable to mock the Request object to a custom one as the Request object is a read-only object. Dynamically setting the header doesn't work, as the Request is null when performing Unit Tests. So the following won't work:

[TestMethod]
public void GenerateTest()
{
    GenerationController target = new GenerationController(this.loginModelMock, this.clipTemplateModelMock, this.clipTemplateFieldModelMock);
    target.Request = this.requestBase;
    string templateId = "0";

    ActionResult actual = target.Generate(templateId);
    Assert.AreEqual(typeof(JsonResult), actual.GetType());
    Assert.AreEqual(typeof(ClipGenerationResponse), ((JsonResult)actual).Data.GetType());
}

Where this.requestBase would be mock created with Moq.

public HttpContextBase CreateMockHttpContext()
{
    var serverVariables = new NameValueCollection {
        { "UserHostAddress", "127.0.0.1" },
        { "UserAgent", "Unit Test Value" }
    };

    var httpRequest = new Moq.Mock<HttpRequestBase>();
    httpRequest.SetupGet(x => x.Headers).Returns(
        new System.Net.WebHeaderCollection {
            {"hash", "somehash"}
        }
    );

    httpRequest.Setup(x => x.ServerVariables.Get(It.IsAny<string>()))
        .Returns<string>(x =>
            {
                return serverVariables[x];
            });

    var httpContext = (new Moq.Mock<HttpContextBase>());
    httpContext.Setup(x => x.Request).Returns(httpRequest.Object);

    return httpContext.Object;
}

Nor the static way would work:

target.Request.Headers["hash"] = "hash";

So, I'm wondering how this could be fixed nicely. I could always obtain the Request in the Constructor, set a class variable to hold the Request, and then mock the getter / setter for testing purposes, but I'd rather use a nicer way to do it. Though I don't seem to know a way to get it working.

Any advice is welcome.

Kind Regards, Lars

PS: Please note that some class names may have been altered for preview.