Allow schema to be nil in Unmarshal

[mfcochauxlaberge]

Currently, the library needs to know about the schema when unmarshaling payloads. With structs like SoftResource, it might be possible to unmarshal an unknown payload with SoftResource and SoftCollection. Validation would be limited and that's probably okay. Providing a schema could simply be a way to add validation and convenience (use the proper structs instead of SoftResource).

Investigate the feasibility and do it if possible.