Current user in WCF REST service hosted in SharePoint consumed by Workflow 2013

I have a custom WCF REST service hosted in SharePoint. It is configured by MultipleBaseAddressWebServiceHostFactory in .svc file:

<%@ ServiceHost 
    Language="C#" 
    Debug="true" 
    Service="In2.Ilas.Sp.Ecase.Svc.PisarnicaService, In2.Ilas.Sp.Ecase, Version=1.0.0.0, Culture=neutral, PublicKeyToken=ac775627f9c7b12a" 
    CodeBehind="Svc/PisarnicaService.cs" 
    Factory="Microsoft.SharePoint.Client.Services.MultipleBaseAddressWebServiceHostFactory, Microsoft.SharePoint.Client.ServerRuntime, Version=15.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c" %>

Service contract is defined as:

[ServiceContract]
public interface IPisarnicaService
{
    [OperationContract]
    Predmet2stIdentification CreateNew(string listName, int idPredmeta1st, string initiator);
}

Service implementation is as follows:

[WebInvoke(
    Method = "POST",
    ResponseFormat = WebMessageFormat.Json,
    BodyStyle = WebMessageBodyStyle.Wrapped,
    UriTemplate = "/CreateNew?listName={listName}&idPredmeta1st={idPredmeta1st}&initiator={initiator}")]
public Predmet2stIdentification CreateNew(string listName, int idPredmeta1st, string initiator)
{
    SPUser user = SPContext.Current.Web.CurrentUser;

    // ...
}   

If I consume this service from webpart with ajax call then user variable will be filled with user who is currently browsing the webpart.

But if I consume same service from Workflow 2013 then user variable will be always filled with application pool account. I would like it to behave the same way as if it was called with ajax call, i.e. to have user variable filled with workflow initiator information. Currently, I am using a workaround where I send workflow initiator information as a string argument to web method.

HttpSend activity has RequestHeaders set up in this way:

Is there a way to automatically propagate workflow initiator information to custom WCF REST service?