Simplified the '@2x' detection and used cache key filter when scaling an image after download

SDWebImageCompat is detecting the "@2x" in a not-that-flexible way. Imagine your image urls uses cache buster, then the range of "len is 8 or more @2x. + 4 len ext" just doesn't work and the scaling of UIImage is set all wrong. Simplifying the detection will just make it more flexible.

On other topic, sometimes the images don't have the '@2x' on the server url, but this doesn't mean it should not be able to respond to the device's density. So using the keyFilter might give some apps to 'fake' the expected behaviour.
This as also been requested by @Tylerc230 for issue #529