Fix writeOnly behaviour

[stolen]

JSON Schema says:

writeOnly indicates that a value may be set, but will remain hidden. In could be used to indicate you can set a value with a PUT request, but it would not be included when retrieving that record with a GET request.

Here I implement writeOnly property filtering on read access, to complement readOnly filtering on write

This changes 'read' access behaviour, so a new access_type is introduce to disable any read/write filtering -- access_type => raw

Additionally, I add a fixture I forgot to add in previous PR #13