Implement -[FLAnimatedImageView intrinsicContentSize]

`UIImageView` returns its image size as its `intrinsicContentSize`, but `FLAnimatedImageView` breaks this when its animatedImage property is set. This change makes `FLAnimatedImageView` much more helpful when using Auto Layout.

I'm not 100% clear why `FLAnimatedImageView`'s behaviour breaks `UIImageView`. Perhaps `UIImageView` bypasses its `image` accessor.