Convert from UIViewAnimationCurve to UIViewAnimationCurveOptions

UIViewAnimationCurve+ToOptions.swift

// Simple function converting from UIViewAnimationCurve to UIViewAnimationCurveOptions
// Not all UIViewAnimationCurve values have a specified equivalent UIViewAnimationCurveOptions,
// making it tricky to use the animation curve of the keyboard with UIView.animateWithDuration(...) API.
// This works as late as iOS 9.1, but could change in the future.

extension UIViewAnimationCurve {
    func toOptions() -> UIViewAnimationOptions {
        return UIViewAnimationOptions(rawValue: UInt(rawValue << 16))
    }
}