a11 metadata link conforms to and schema org

[danielweck]

https://github.com/standardebooks/oscar-wilde_an-ideal-husband/blob/4a454ad572188d210ea2d26c4004033694b0ec3c/src/epub/content.opf#L19-L28

http://readium2.herokuapp.com/pub/aHR0cHM6Ly9zdGFuZGFyZGVib29rcy5vcmcvZWJvb2tzL29zY2FyLXdpbGRlL2FuLWlkZWFsLWh1c2JhbmQvZG93bmxvYWRzL29zY2FyLXdpbGRlX2FuLWlkZWFsLWh1c2JhbmRfYWR2YW5jZWQuZXB1Yg%3D%3D/manifest.json/show/all

[danielweck]

Incorrect parsing:

https://github.com/IDPF/epub3-samples/blob/main/30/accessible_epub_3/EPUB/package.opf

http://readium2.herokuapp.com/pub/aHR0cHM6Ly9yYXcuZ2l0aHVidXNlcmNvbnRlbnQuY29tL0lEUEYvZXB1YjMtc2FtcGxlcy9tYWluLzMwL2FjY2Vzc2libGVfZXB1Yl8zL01FVEEtSU5GL2NvbnRhaW5lci54bWw%3D/manifest.json/show/all