Fixed the issue in serialization of emoji chars

On serializing a text with emoji in it (Text with 😜 emoji 🎉), using the code (running on Mono framework ver 3.2.5):

```
var writer = new JsonFx.Json.JsonWriter ();
string json = writer.Write (objectToSerialize);
```

the library threw an exception: "The string contains invalid surrogate pair character at….". 

This has been fixed in this pull request.