title: SDWebImage refreshing
body: <p>I am using the <code>SDWebImage</code> to load the image from URL. When image is updated on the server, then image is not updating in the image view even i have used <code>options: .refreshCached</code>. After updating image, URL remains same. 
  Can i directly change image in cache for this URL?
  I googled for this question,i got that i can not change it in cache directly. If i can not change image in cache directly for specify URL, Is it possible to clear cached value for specific key (URL)?</p>
  
  <pre><code>   self.imgView.sd_setImage(with: url, placeholderImage: #imageLiteral(resourceName: "Timg"), options: .refreshCached, completed: { (image: UIImage?, error: Error?, cacheType: SDImageCacheType, imageURL: URL?) in
                  if image != nil{
                      self.imgView.contentMode = .scaleAspectFill
                  }
              })
  </code></pre>