Fix `sd_imageFormat` sometimes returns `undefined`

[lukmccall]

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This merge request fixes: sd_imageFormat returns nil when used on SDAnimatedImage to which static image were loaded.

Pull Request Description

When a static image is loaded into SDAnimatedImage, the sd_imageFormat remains unset. As a result, the associated object on the underlying UIImage is not present. This value is typically assigned by decoders; for instance, SDImageIOCoder does so here. However, with SDAnimatedImage, the correct format is returned only if the animatedImageData is included.

I’m uncertain if my solution is accurate (I’ve tested it, but I’m unsure if it’s the right approach). I’ve opened this PR to initiate a discussion on how to properly address that issue.