Rewrite mastodon-http--get-json to return a list including header info

[alexjgriffith]

mastodon-http--get-json currently returns a vector, it can be adapted to return a list.

While these changes are being made I recommend adding header info to the output. This will be required for allowing nav in favourites and other timelines that are sorted using an internal key. Something like:

'((("Server" ."Mastodon")
   ("Link" "<https://mastodon.social/api/v1/favourites?max_id=6987771>; rel=\"next\", <https://mastodon.social/api/v1/favourites?since_id=7010142>; rel=\"prev\")
    ... )  
  . (Body-ElementA Body-ElementB ...))

From documentation
Note: max_id and since_id for next and previous pages are provided in the Link header. It is not possible to use the id of the returned objects to construct your own URLs, because the results are sorted by an internal key.

HTTP/1.1 200 OK
Date: Sat, 10 Mar 2018 18:58:37 GMT
Content-Type: application/json; charset=utf-8
Transfer-Encoding: chunked
Connection: keep-alive
Server: Mastodon
X-Frame-Options: DENY
X-Content-Type-Options: nosniff
X-XSS-Protection: 1; mode=block
X-RateLimit-Limit: 300
X-RateLimit-Remaining: 298
X-RateLimit-Reset: 2018-03-10T19:00:00.411404Z
Link: <https://mastodon.social/api/v1/favourites?max_id=6987771>; rel="next", <https://mastodon.social/api/v1/favourites?since_id=7010142>; rel="prev"
Vary: Accept-Encoding, Origin
Content-Encoding: gzip
ETag: W/"e11011ad7071cbd13fea1f37aaeeaf18"
Cache-Control: max-age=0, private, must-revalidate
X-Request-Id: 466b636a-9c5a-4bb6-9e06-cf6b42c5ab4f
X-Runtime: 0.152353
Strict-Transport-Security: max-age=31536000