JSON.NET Serializer for WCF REST Services

I am trying to use the NETFx Json.NET MediaTypeFormatter nuget package to swap out the default DataContractJsonSerializer in my WCF REST service (4.0 framework). I downloaded the package in my project and added the following lines of code in the Global.asax file.

    void Application_Start(object sender, EventArgs e)
    {
        RegisterRoutes();

        // Create Json.Net formatter serializing DateTime using the ISO 8601 format
        var serializerSettings = new JsonSerializerSettings();
        serializerSettings.Converters.Add(new IsoDateTimeConverter());

        var config = HttpHostConfiguration.Create();
        config.Configuration.OperationHandlerFactory.Formatters.Clear();
        config.Configuration.OperationHandlerFactory.Formatters.Insert(0, new JsonNetMediaTypeFormatter(serializerSettings));
    }

But when I run the service it still uses the DataContractJsonSerilizer for serialization. Below is the class I am returning from my service.

[DataContract]
public class SampleItem
{
    [DataMember]
    public int Id { get; set; }

    [DataMember]
    public string StringValue { get; set; }

    [DataMember]
    public DateTime DateTime { get; set; }
}

Below is the response from the service in Fiddler.

You can see that the DateTime is not in ISO format which I have specified in serializerSettings in the above code. This tells me that the JSON.NET serializer is not used for serializing the objects.

Would appreciate any help.