Add a method to copy a cached image if it exists

Sometimes you (think you might) have an image in the cache and you want to copy that image to a new key. For example you have an image cached locally under a local URL, and then upload the image to a server. The URL then changes to a remote URL hitting the server. If you re-fetch the image, then you end up with a short period of time while the image is re-downloading where you think you don't have the image for that key, but really, you do.

This adds a method that allows you to copy the image if it exists for a certain key to another key.