即宽度不够时候，拉升到屏幕宽度，然后保持宽高比不变：

iv.snp_makeConstraints { [unowned contentView] (make) in
     if let image = iv.image where image.size.width > 0 {
        make.height.equalTo(image.size.height * (screenWidth - CGFloat(padding * 2)) / image.size.width)
    }
}