Unit test for 'Link' header

[jeroen]

The jsonld spec section 6.8 Interpreting JSON as JSON-LD defines a special case where the context of a regular json file served over http can be set in the Link http response header:

Ordinary JSON documents can be interpreted as JSON-LD by referencing a JSON-LD context document in an HTTP Link Header. Doing so allows JSON to be unambiguously machine-readable without requiring developers to drastically change their documents and provides an upgrade path for existing infrastructure without breaking existing clients that rely on the application/json media type or a media type with a +json suffix as defined in [RFC6839].

The R package should support this but I would like to add a unit test to actually confirm that it works. However I can't find any example server that uses this.

@cboettig @maelle