title: Firebase Database for Unity3d Android ValueChanged unsubscribe issues
body: <p>I have problem with Unity3d SDK.
  I subscribe to ValueChanged event and when first time it invokes I unsubscribe from the handler.
  In Unity everything works fine.
  But when I build project for Android, unsubscribe fails: after unsubscribe I still handle events in handler.
  Here is the code:</p>
  
  <pre><code>public void CheckUserExists()
  {
      FirebaseDatabase.DefaultInstance
          .GetReference("users").Child(primary_key)
          .ValueChanged += CheckUserExistsHadler;
  }
  
  void CheckUserExistsHadler(object sender, ValueChangedEventArgs e)
  {
      FirebaseDatabase.DefaultInstance
          .GetReference("users").Child(primary_key)
          .ValueChanged -= CheckUserExistsHadler;
  
      if (e.Snapshot == null || e.Snapshot.Value == null)
      {
          print("user_exists");
      }
      else
      {
          print("user_not_exists");
      }
  }
  </code></pre>