Fix issue with lax schemas

[stefandesu]

Lax schemas (i.e. option unknownFields is set to true) don't work correctly. I'm going to add a simple failing test case to the dev branch.

[stefandesu]

Fixed in https://github.com/gbv/jskos. The issue was that oneOf instead of anyOf was used, and for my particular example, there were now two options fulfilled in the oneOf array, causing it to fail instead.