After creation a resource is not rendered if its id is nil

In 0.8.3, I could 'create' a PORO object, and let the id of the object be nil, and everything would render well, i.e.:

class Poro
  def id
    nil
  end

  def save
    # some logic
  end
end

class PoroResource < JSONAPI::Resource; end

# POST /poros
{ "data": { "attributes": {}, "type": "poros" } }

# Response
{ "data": { "attributes": {}, "type": "poros", "id": null, "..etc..": "..etc.." } }

However, in 0.9.0, the data is just null:

# POST /poros
{ "data": { "attributes": {}, "type": "poros" } }

# Response
{ "data": null }

This is due to https://github.com/cerebris/jsonapi-resources/blob/release-0-9/lib/jsonapi/resource_serializer.rb#L69

Maybe a short line in the docs somewhere would let people know this happens.

Start with the behavior described for POST /poros and inspect jsonapi/resource_serializer.rb at the referenced line in the release-0-9 branch. Confirm how a nil resource id affects serialization, then document the observed behavior so users know what to expect.