El formateador Scala no puede serializar una cadena json que distingue entre mayúsculas y minúsculas

I wrote my own play.api.libs.json.Format for a class i am read/writing as JSON.

My goal is to build the following class:

WallpaperItem(title:String, filename:String, tileSize:Option[String], url:Option[String])

The JSON formatter looks like:

val wallpaperItemFormat= new Format[WallpaperItem] {
 def writes(data: WallpaperItem): JsValue = {
  Json.obj(
      "title" -> data.title,
      "filename" -> data.filename,
      "tileSize" -> data.tileSize,
      "url" -> data.url
      )}
def reads(json: JsValue): JsResult[WallpaperItem] = {
  JsSuccess(new WallpaperItem(
      (json \ "title").as[String],
      (json \ "filename").as[String],
      (json \ "tileSize").as[Option[String]], 
      (json \ "url").as[Option[String]]
   ))}
} 

The JSON string in debug is:

{
    "title": "MILANO STRIA",
    "filename": "MS21-74.jpg",
    "tileSize": 32,
    "url": "http://www.koroseal.com/images/designs_large/MS21-74.jpg"
}