better spine itemref's properties attribute handling

[skoji]

Spine itemref element's properties attribute holds data like page-spread or rendition setting in fixed layout data. Gepub supports to read and write properties attribute of itemref elements as raw string, so a client should parse the string manually.

[127]

By the way, is there any simple way to set rendition type? It's terrifying hard now — get package.opf from epub — reassemble it with nokogiri and pack it back.