Incomplete downloaded image is cached.

## 🐛 Bug Report

I have encountered an issue where a lot of images is not completely downloaded, and is partly gray. It looks like only 10% of the image is downloaded. And the problem is that, the incompleted image is also cached in the memory, and it won't download again.
How should I fixed it?

**Version: latest **

**Platform:** 
- Xamarin iOS
