OkHttp If-None-Match header is missing

Question

I'm creating an Android app in which I use OkHttp networking library as an HTTP client. I share a single OkHttpClientinstance with a cache size of 10 MB. The flow is: I send a request to a server, the server handles it and produces the ETag header, which OkHttp is supposed to hook up into the If-None-Match header in the following requests to the same URL. The problem is that If-None-Match header is missing from request's headers and, therefore, Response.cacheResponse() always returns null. So my question is: how do I fix that to use response caching?

OkHttpClientSingleton.java:

public class OkHttpClientSingleton
{
   private static OkHttpClientSingleton sOkHttpClientSingleton;

   private OkHttpClient mOkHttpClient;

   public static OkHttpClientSingleton get(Context context)
   {
       if(sOkHttpClientSingleton == null)
       {
           sOkHttpClientSingleton = new OkHttpClientSingleton(context);
       }

       return sOkHttpClientSingleton;
   }

   public OkHttpClientSingleton(Context context)
   {
       int cacheSize = 10 * 1024 * 1024;
       Cache cache = new Cache(context.getCacheDir(), cacheSize);

       mOkHttpClient = new OkHttpClient.Builder()
               .cache(cache)
               .addNetworkInterceptor(new LoggingInterceptor())
               .build();
   }

   public void addRequest(Request request, Callback responseCallback, ProgressDialog progressDialog)
   {
       if(progressDialog != null && !progressDialog.isShowing())
           progressDialog.show();

       mOkHttpClient.newCall(request).enqueue(responseCallback);
   }
}

Sending a request code snippet:

RequestBody formBody = new FormBody.Builder()
                    .add("data", data)
                    .build();

Request request = new Request.Builder()
                    .url(url)
                    .post(formBody)
                    .build();

Callback responseCallback = new Callback()
{
    @Override
    public void onFailure(Call call, IOException e)
    {
        // Do something
    }

    @Override
    public void onResponse(Call call, Response response)
    {
        dismissProgressDialog();

        Gson gson = new GsonBuilder()
                      .setFieldNamingPolicy(FieldNamingPolicy.LOWER_CASE_WITH_UNDERSCORES)
                      .create();

        Response cacheResponse = response.cacheResponse(); // Always null

        if(response.isSuccessful())
        {
            if(response.code() == 200)
            {                   
                // Deserialize json using gson
            }
        }
        else
        {
            if(response.code() == 400)
            {
                // Deserialize json using gson
            }
        }
    }
};

OkHttpClientSingleton.get(getContext()).addRequest(request, responseCallback, getProgressDialog());

Logs from a network interceptor for two identical requests can be seen here.

I've found quite a lot similar questions about it on the Internet, but none of them helped me to solve this problem.

Any help would be much appreciated.