[rdf] Fix JSON-LD type serialization

[ddeboer]

We used to produce JSON-LD documents like:

"@type": {
    "@id": "http://..."
}

However, this doesn't validate against https://json-ld.org/playground/, so change it to:

"@type": ["http://..."]

which does validate.

[rl-king]

@ddeboer We're about to tag a new release and while reviewing the changes we were wondering what impact of this change is to existing code. Won't all code expecting the existence of "@id" break?

[ddeboer]

Hey @rl-king! You’re right, this is definitely a BC break. In practice:

• internal callers of the affected ginger_json_ld:serialize_to_map/1 are mainly controller_rdf:rdf/2 and m_rdf:m_find_value/3; they serialize straight to JSON (which still works fine)
• any external JSON-LD parsers were unable to parse @type before anyway because it was not conforming to the JSON-LD standard.

[rl-king]

👍 thanks for the quick response!