Made jsonmapper for dictionary use InvariantCulture

Author: grantfar

src/LitJson/JsonMapper.cs

@@ -779,10 +779,13 @@ private static void WriteValue (object obj, JsonWriter writer,
                 return;
             }
 
-            if (obj is IDictionary) {
+            if (obj is IDictionary dictionary) {
                 writer.WriteObjectStart ();
-                foreach (DictionaryEntry entry in (IDictionary) obj) {
-                    writer.WritePropertyName (entry.Key.ToString());
+                foreach (DictionaryEntry entry in dictionary) {
+                    var propertyName = entry.Key is string key ?
+                        key
+                        : Convert.ToString(entry.Key, CultureInfo.InvariantCulture);
+                    writer.WritePropertyName (propertyName);
                     WriteValue (entry.Value, writer, writer_is_private,
                                 depth + 1);
                 }