title: Unity C# JsonUtility is not serializing a list
body: <p>I've got some data I need to serialize/deserialize, but JsonUtility is just not doing what it's supposed to. Here's the objects I'm working with:</p>
  
  <pre><code>public class SpriteData {
      public string sprite_name;
      public Vector2 sprite_size;
      public List&lt;Vector2&gt; subimage;
  }
  
  public class SpriteDataCollection
  {
      public SpriteData[] sprites;
  }
  </code></pre>
  
  <p>If I create a SpriteDataCollection, and attempt to serialize it with JsonUtility, I just get an empty object {}. Here's how it's being built:</p>
  
  <pre><code>            SpriteData data = new SpriteData();
              data.sprite_name = "idle";
              data.sprite_size = new Vector2(64.0f, 64.0f);
              data.subimage = new List&lt;Vector2&gt; { new Vector2(0.0f, 0.0f) };
  
              SpriteDataCollection col = new SpriteDataCollection();
              col.sprites = new SpriteData[] { data };
  
              Debug.Log(JsonUtility.ToJson(col));
  </code></pre>
  
  <p>The debug log only prints "{}". Why isn't it serializing anything? I've tested it out, and serializing a single SpriteData does exactly what it's supposed to do, but it won't work in the SpriteDataCollection.</p>