title: rightCalloutAccessoryView not shown on the MKPinAnnotationView
body: <p>I use the MKPinAnnotationView on the MKMapView, <strong>the title and subtitle is shown normally</strong>, but the <code>rightCalloutAccessoryView</code> is not shown.</p>
  
  <p>I tried google a lot, but could not show it yet. My code is below.</p>
  
  <pre><code>- (void)addAnnotation
  {
      CLLocationCoordinate2D theCoordinate = self.location.coordinate;
  
      MapAnnotation *annotation = [[MapAnnotation alloc] initWithCoordinate:theCoordinate];
      annotation.title = @"Pin";
      annotation.subtitle = @"More information";
  
      [self.mapView addAnnotation:annotation];
  }
  
  - (MKAnnotationView *)mapView:(MKMapView *)mapView viewForAnnotation:(id&lt;MKAnnotation&gt;)annotation
  {
      if ([annotation isKindOfClass:[MapAnnotation class]]) {
          static NSString *reuseIdentifier = @"MyMKPinAnnotationView";
          MKPinAnnotationView *annotationView = (MKPinAnnotationView *)[self.mapView dequeueReusableAnnotationViewWithIdentifier:reuseIdentifier];
  
          if (!annotationView) {
              annotationView = [[MKPinAnnotationView alloc] initWithAnnotation:annotation reuseIdentifier:reuseIdentifier];
              annotationView.pinColor = MKPinAnnotationColorRed;
              annotationView.enabled = YES;
              annotationView.canShowCallout = YES;
              annotationView.rightCalloutAccessoryView = [UIButton buttonWithType:UIButtonTypeDetailDisclosure];
          } else {
              annotationView.annotation = annotation;
          }
      }
  
      return nil;
  }
  </code></pre>
  
  <p>Notice that title &amp; subtitle can be shown.</p>
  
  <p>Thanks in advance.</p>