title: autolayout's visual format does not work with iOS8
body: <p>I have a simple <code>ViewController</code> that contains only its default view with a background image placed on it, I have the following <code>updateViewConstraints</code> method which works perfectly on <code>iOS 7</code> devices (without any call to setNeedsXXX methods):</p>
  
  <pre><code>    -(void) updateViewConstraints {
  
  [super updateViewConstraints];
  
  [self.view removeConstraints:self.view.constraints];
  NSDictionary *viewsDict = @{@"bgImage":self.bgImageView};
  
  NSArray * constraints;
  constraints = [NSLayoutConstraint constraintsWithVisualFormat:@"H:|[bgImage]|" options:0 metrics:nil views:viewsDict];
  [self.view addConstraints:constraints];
  
  constraints = [NSLayoutConstraint constraintsWithVisualFormat:@"V:|[bgImage]|" options:0 metrics:nil views:viewsDict];
  [[self.view addConstraints:constraints];
  
  }
  </code></pre>
  
  <p>however, this method does not work as expected on <code>iOS 8</code> devices, the background image is shifted up and didn't fit in place.</p>
  
  <p>any ideas would be highly appreciated.</p>