Use attributeNS for getting authors' file-as attribute

[rschroll]

In 8cb6dcdf, we get the opf:file-as attribute to use for sorting the authors name. But I don't think there's any guarantee that the NS prefix will be opf. Instead, we should use attributeNS with the NS URI (http://www.idpf.org/2007/opf), but that isn't working in my testing.

We should find out what's wrong and fix it if possible.