Change MessageStyle.bubbleImageCache access level to internal
and extend unit test to clear cache and check for inequality
after an image is requested again
Cache stretchable bubble image data in memory using NSCache.
UIImage(contentsOfFile:) does _not_ cache the resulting image
so it needs to be handled explicitly, and there is no need to
reload the image from disk for every message.
zhongwuzw
Nick Donaldson
Rich Gabrielli
Candost Dagdeviren