How do Test HttpWebRequest Depended method?

I have this class:

public class RestClient
{
    public RestClient() 
    { }

    protected virtual HttpWebRequest CreateHttpWebRequest(Uri uri)
    {
        return (HttpWebRequest)HttpWebRequest.Create(uri);
    }


    /// <summary>
    /// Perform a http POST request in order to push data to server
    /// </summary>
    /// <param name="uri">End Point Uri</param>
    /// <param name="data">Data to be transmitted</param>
    /// <returns></returns>
    /// 
    public long PostRequest(Uri uri,string data) 
    {

        try
        {
            HttpWebRequest request = CreateHttpWebRequest(uri); //(HttpWebRequest)HttpWebRequest.Create(uri);

            request.Method = "POST";
            request.ContentType = "application/json";

            System.Text.UTF8Encoding encoding = new UTF8Encoding();

            byte[] bytes = encoding.GetBytes(data);



            using (Stream requestStream = request.GetRequestStream())
            {
                //Transmit data
                requestStream.Write(bytes, 0, bytes.Length);
                requestStream.Flush();
                requestStream.Close();
            }



            //Get the Response from the server
            using (HttpWebResponse response = (HttpWebResponse)request.GetResponse())
            {
                if (response.StatusCode != HttpStatusCode.OK && response.StatusCode != HttpStatusCode.NoContent)
                {
                    throw new Exception(String.Format(
                       "Server error (HTTP {0}: {1}).",
                            response.StatusCode,
                                response.StatusDescription));
                }
            }


            return request.ContentLength;
        }
        catch (Exception e)
        {
            throw e;
        }

    }       

}

And would like to Unit test (using nunit) the PostRequest Method.

Doing some research, I could found some way to mock the HttpWebRequest in this post (Is it possible to mock out a .NET HttpWebResponse?) and a way to inject it into the class in this post (How to unit test a method with HttpWebRequest/Response dependencies).

However, when I tried to test my method I got this error:

System.InvalidCastException : Unable to cast object of type 'Castle.Proxies.IHttpWebRequestProxy' to type 'System.Net.HttpWebRequest'.

in this line of my test

client.HttpWebRequestFake = (HttpWebRequest)factory.Object.Create("http://127.0.0.1");

That is my test code:

public class TesableRestClient : RestClient
{
    public HttpWebRequest HttpWebRequestFake { get; set; }

    protected override HttpWebRequest CreateHttpWebRequest(Uri url)
    {
        if (HttpWebRequestFake != null)
            return HttpWebRequestFake;
        return base.CreateHttpWebRequest(url);
    }
}


[TestFixture]
public class TransferWebRequestTest
{
    [Test]
    public void TestPostResquest()
    {
        string expectedContent  = "Content";
        var expectedBytes = Encoding.UTF8.GetBytes(expectedContent);
        var responseStream = new MemoryStream();
        responseStream.Write(expectedBytes, 0, expectedBytes.Length);
        responseStream.Seek(0, SeekOrigin.Begin);

        var response = new Mock<IHttpWebResponse>();
        response.Setup(c => c.GetResponseStream()).Returns(responseStream);

        var request = new Mock<IHttpWebRequest>();
        request.Setup(c => c.GetResponse()).Returns(response.Object);

        var factory = new Mock<IHttpWebRequestFactory>();
        factory.Setup(c => c.Create(It.IsAny<string>()))
            .Returns(request.Object);

        TesableRestClient client = new TesableRestClient();
        client.HttpWebRequestFake = (HttpWebRequest)factory.Object.Create("http://127.0.0.1");

        // DoStuff call the url with a request and then processes the
        long bytesSent = client.PostRequest(new Uri("http://127.0.0.1"), expectedContent);
         Assert.AreEqual(expectedBytes, bytesSent);
}

The HttpWebRequest/Response is this:

public interface IHttpWebRequest
{
    // expose the members you need
    string Method { get; set; }
    string ContentType { get; set; }
    long ContentLength { get; set; }
    IHttpWebResponse GetResponse();
}

public interface IHttpWebResponse : IDisposable
{
    // expose the members you need
    HttpStatusCode StatusCode { get; }
    string StatusDescription { get;}
    Stream GetResponseStream();
}

public interface IHttpWebRequestFactory
{
    IHttpWebRequest Create(string uri);
}

// barebones implementation

public class HttpWebRequestFactory : IHttpWebRequestFactory
{
    public IHttpWebRequest Create(string uri)
    {
        return new WrapHttpWebRequest((HttpWebRequest)WebRequest.Create(uri));
    }
}

public class WrapHttpWebRequest : IHttpWebRequest
{
    private readonly HttpWebRequest _request;

    public WrapHttpWebRequest(HttpWebRequest request)
    {
        _request = request;
    }

    public string Method
    {
        get { return _request.Method; }
        set { _request.Method = value; }
    }

    public string ContentType
    {
        get { return _request.ContentType; }
        set { _request.ContentType = value; }
    }

    public long ContentLength
    {
        get { return _request.ContentLength; }
        set { _request.ContentLength = value; }
    }

    public IHttpWebResponse GetResponse()
    {
        return new WrapHttpWebResponse((HttpWebResponse)_request.GetResponse());
    }
}

public class WrapHttpWebResponse : IHttpWebResponse
{
    private HttpWebResponse _response;

    public WrapHttpWebResponse(HttpWebResponse response)
    {
        _response = response;
    }

    public void Dispose()
    {
        Dispose(true);
        GC.SuppressFinalize(this);
    }

    private void Dispose(bool disposing)
    {
        if (disposing)
        {
            if (_response != null)
            {
                ((IDisposable)_response).Dispose();
                _response = null;
            }
        }
    }

    public Stream GetResponseStream()
    {
        return _response.GetResponseStream();
    }

    public HttpStatusCode StatusCode
    {
        get { return _response.StatusCode; }
    }

    public string StatusDescription
    {
        get { return _response.StatusDescription; }
    }

}

Any idea of how I could solve this?

Thank you