title: can't set zoom level on MKMapView
body: <p>i am adding <code>MKCircleView</code> to the user annotation like so : </p>
  
  <pre><code>- (void)mapView:(MKMapView *)mapView didUpdateUserLocation:(MKUserLocation *)userLocation
  {
  if (!_MapCentered) {
      **_circle = [MKCircle circleWithCenterCoordinate:userLocation.coordinate radius:3000];
      [_map_view addOverlay:_circle];** 
      _MapCentered = YES;
      }
  }
  </code></pre>
  
  <p>it will fire once and once the user location has traced, it works well but as you can see the diameter of the circle view is 3000 meters. so now i want the zoom level to fit the <code>CircleView</code> like so :</p>
  
  <pre><code>        MKCoordinateRegion viewRegion = MKCoordinateRegionMakeWithDistance(mapView.userLocation.coordinate, 0.270, 0.270);
      [_map_view setRegion:viewRegion animated:YES];
  </code></pre>
  
  <p>i have changed the delta degrees to other numbers but nothing is changed.
  how can i manage this?</p>