Handle JSON-LD

[jeff-zucker]

We need to

• check that the correct content type is associated with json-ld files
• handle the Accept header in requests
  • send turtle/json depending on header
  • reject requests for wrong type
• make sure response.json() works as expected

[jeff-zucker]

JSON-LD works, but we don't examine the Accept header, only send JSON-LD for *.jsonld files.