djatoka_region method should set the Djatoka::Region @rft_id property

This commit fixes a problem with Djatoka::IiifRequest.djatoka_region in which the identifier of the image is not set correctly. (There is no :id symbol in the @iiif_params hash of the Djatoka::IiifRequest object -- the image id is actually in the @id attribute of the Djatoka::IiifRequest object.)