Set a @base before json-ld -> rdf nquads

[cboettig]

Otherwise jsonld::jsonld_to_rdf will drop triples if the id is not a URI. Setting a @base to the document URL / document path is the standard behavior (e.g. in json-ld.org/playground, or python rdflib), see ropensci/jsonld#9.

Not obvious what this base should be. One option is the anonymous id "@base": "_:", which will replace the local ids with ids like JSON-LD does for anonymous objects (elements that didn't declare an @id), i.e. _:b0, _:b1, etc. This destroys the original string used as the id, which semantically shouldn't matter but is probably not nice all the same.

Other libraries just set the base to the file path, i.e. "@base": "some:url/or/local/path", which seems a bit arbitrary, particularly when it's exposing full user paths in the data somewhat surreptitiously. Maybe a default like "@base": "doc:" would be best.

[cboettig]

https://github.com/ropensci/jsonld/blob/9a5dcb1b1e04aee218dc1dead06fa0bcc489ee1e/inst/js/jsonld.js#L889-L896