fix(image-loader): queue stucks processing same url more than concurrency
fix(image-loader): queue stucks processing same url more than concurrency

When loading the same url more than the allowed by 'concurrency', the
queue may gets stuck, because the logic to prevent same image from loading
at same time doesn't release the 'processing' counter , which is
incremented always, even when processing a url that is already being
processed.

zyra/ionic-image-loader,zyramedia/ionic-image-loader