#50 broke serialization

[kasei]

50 seems to have broken serialization in cases where there are multiple blank node objects being serialized. The behavior makes me think the serializer stores '' as an "identifier" (scare quotes) that it has already handled, and then when it finds a new blank node, it thinks it's already done, so emits just the bare minimum.

In this example, the content of the second identifier just disappears upon serialization:

>>> i1 = vocab.IsbnIdentifier(ident='', content='0000')
>>> i2 = vocab.IsbnIdentifier(ident='', content='9999')
>>> s = vocab.Note()
>>> s.identified_by = i1
>>> s.identified_by = i2
>>> print(factory.toString(s, False))
{
  "@context": "https://linked.art/ns/v1/linked-art.json",
  "id": "http://lod.example.org/museum/LinguisticObject/1",
  "type": "LinguisticObject",
  "classified_as": [
    {
      "id": "http://vocab.getty.edu/aat/300027200",
      "type": "Type",
      "_label": "Note"
    },
    {
      "id": "http://vocab.getty.edu/aat/300418049",
      "type": "Type",
      "_label": "Brief Text"
    }
  ],
  "identified_by": [
    {
      "type": "Identifier",
      "content": "0000",
      "classified_as": [
        {
          "id": "http://vocab.getty.edu/aat/300417443",
          "type": "Type",
          "_label": "ISBN Identifier"
        }
      ]
    },
    {
      "type": "Identifier"
    }
  ]
}

[azaroth42]

Fixed by #52.