How to test WP app with web requests?

I am developing windows phone application, where data comes from web. I would like to test some method:

public void PrintSomeObject()
{
    var apiInstance = new Api();
    apiInstance.GetSomeObject("bar",
    (res) => 
    {
        Debug.WriteLine(res.data);
    });
}

public class Api
{
    public GetSomeObject(string path, Action<SomeObject> callback,)
    {
        HTTPRequest("http://foo.com/" + path,
        (resultStr) =>
        {
            SomeObject t = ParseSomeObject(resultStr);
            callback(t);            
        });
    }

    void async void HTTPRequest(string baseUri, Action<string> resultCallback)
    {
        var result = await httpClient.PostAsync(new Uri(baseUri, UriKind.Absolute), content);
        var resultStr = await result.Content.ReadAsStringAsync();
        // var resultStr = "{data: 'some fake data', number: 42}" // I want insert fake data here
        resultCallback(resultStr);
    }
}

I know want PrintSomeObject() should print in some cases, and I'd like to test it. But I need to control data which I get in request response. Is it possible, or I need test each method separatly?