Unit testing HTTP requests

I would like to unit test my web requests, so on top of my head I came up with the solution below.

Since it seems to be working fine, I'm posting it on this site. Though it doesn't feel very dynamic, I would like your input. For example, this solution can't simulate response headers, which I'm also interested in mocking:

public interface IWebRequest
{
    Task<string> DownloadString();
}

public class RealWebRequest : IWebRequest
{
    private readonly string url;

    public RealWebRequest(string url)
    {
        this.url = url;
    }

    public Task<string> DownloadString()
    {
        using (WebClient client = new WebClient())
        {
            return client.DownloadStringTaskAsync(new Uri(url));
        }
    }
}

public class FakeWebRequest : IWebRequest
{
    private readonly string fakeReponse;

    public FakeWebRequest(string fakeReponse)
    {
        this.fakeReponse = fakeReponse;
    }

    public Task<string> DownloadString()
    {
        return Task.Factory.StartNew(() => fakeReponse);
    }
}

public class TestClass
{
    private readonly IWebRequest request;

    public TestClass(IWebRequest request)
    {
        this.request = request;
    }

    public async Task<string> DoStuff()
    {
        return await request.DownloadString();
    }
}

"Real" usage example:

IWebRequest realRequest = new RealWebRequest("http://www.test.com");
TestClass test = new TestClass(realRequest);
string realResult = await test.DoStuff();

Unit test usage example:

IWebRequest fakeRequest = new FakeWebRequest("fake response string");
TestClass test = new TestClass(fakeRequest);
string fakeResponse = await test.DoStuff();