C# ServiceBehavior custom json serialization

Question

When deserializing a request I get a non-null but empty dictionary.

Object I want to post:

public class Data
{
    public IDictionary<string, object> Dictionary { get; set; }
}

Here is the body I send with my request:

{"Dictionary":{"key1":"value1","foo":"bar"}}

The built-in serializer creates a Data object with an empty Dictionary. The Newtonsoft serializer behaves how I want. So after some googling I changed the service contract to accept a Stream rather than a Data.

using Newtonsoft.Json;

[ServiceContract]
interface IMyServiceContract
{
    [OperationContract]
    [WebInvoke(
        Method = "POST",
        UriTemplate = "data",
        RequestFormat = WebMessageFormat.Json)]
    void PostData(Stream body);
}

class MyServiceContract : IMyServiceContract
{
    void PostData(Stream body)
    {
        using (var reader = new StreamReader(body))
            json = reader.ReadToEnd();
        data = JsonConvert.DeserializeObject<Data>(json);
        // ...
    }
}

The main problem here is that the request is no longer accepted if I specify the header Content-Type: application/json, which I obviously want, but it would also be nice if the method signature mentioned Data.

How can I specify a custom deserializer for my service? Or, if not possible, make the current solution work even if Content-Type is specified?