Added the default aspect ratio setting

[bratyslav]

Added the default image aspect ratio setting to correctly initialize the ISVImageScrollView instance, even if imageView has not yet been set.

[yuriiik]

Hi @bratyslav!
Thank you for your pull request. Could you please provide an example when one might need defaultAspectRatio property?

[bratyslav]

Yes, I use the ISVImageScrollView to show the camera preview with a zoom option, but in a viewDidLoad method of a UIViewController the camera had not been yet initialized.
Or, for instance, you might wait for an image to be loaded from a server, etc.
So, you might know a ratio of the frame, but there is actually no frame at the moment of initialization.

[yuriiik]

I think I reproduced the issue. UIImageView gets incorrect frame when it's assigned to ISVImageScrollView while its image property is nil.

  override func viewDidLoad() {
    super.viewDidLoad()
    self.imageView = UIImageView(image: nil)
    self.imageScrollView.imageView = self.imageView
    DispatchQueue.main.async {
      self.imageView?.image = UIImage(named: "Photo.jpg")
    }
    self.imageScrollView.maximumZoomScale = 4.0
    self.imageScrollView.delegate = self
  }

To make it work as expected just make sure that UIImageView has a non-nil image before assigning it to the ISVImageScrollView.

  override func viewDidLoad() {
    super.viewDidLoad()
    self.imageView = UIImageView(image: nil)
    DispatchQueue.main.async {
      self.imageView?.image = UIImage(named: "Photo.jpg")
      self.imageScrollView.imageView = self.imageView
    }
    self.imageScrollView.maximumZoomScale = 4.0
    self.imageScrollView.delegate = self
  }

Does this work for you?