override accept or consistent behavior

Author: mythz

src/ServiceStack.Text/HttpUtils.HttpClient.cs

@@ -1086,7 +1086,10 @@ public static HttpRequestMessage With(this HttpRequestMessage httpReq, Action<Ht
         var headers = config.Headers;
 
         if (config.Accept != null)
-            headers.Add(new(HttpHeaders.Accept, config.Accept));
+        {
+            httpReq.Headers.Accept.Clear(); //override or consistent behavior
+            httpReq.Headers.Accept.Add(new(config.Accept));
+        }
         if (config.UserAgent != null)
             headers.Add(new(HttpHeaders.UserAgent, config.UserAgent));
         if (config.ContentType != null)