title: Class won't serialize into JSON using unity's JSONUtility
body: <p>I have the following class, which is serializable and only has strings as fields:</p>
  
  <pre><code>using UnityEngine;
  using System.Collections;
  using System.Collections.Generic;
  
  [System.Serializable]
  public class Cabeza
  {
  
      string id, urlOBJ, urlTextura, pathOBJbajado, pathTexturaBajada;
  
      public string Id { get; set; }
      public string UrlOBJ { get; set; }
      public string UrlTextura { get; set; }
      public string PathOBJbajado { get; set; }
      public string PathTexturaBajada { get; set; }
  
  
      public Cabeza (string nuevoId)
      {
          Id = nuevoId;
          UrlOBJ =  nuevoId +".obj";
          UrlTextura =  nuevoId + ".png";
  
      }
  
  
  }
  </code></pre>
  
  <p>As far I know it should be possible to obtain a JSON from it...However, JsonUtility.ToJson() returns just <code>{ }</code>. How is this possible? What am I missing?</p>