Not quite sure if the content negotiation is part of the extension or the site setup. I think it happen after the first hit on the Special:EntityData, and therefore is part of the extension.
During negotiation between the external client (as in web browser) and the repo the sequence is now HTTP 301 (secure connection) - HTTP 303 (address resolution) - HTTP 303 (content negotiation) - HTTP 200 (content delivery).
The response code for address resolution should be HTTP 303 See other (correct impl., Cool URIs for the Semantic Web) while content negotiation should be HTTP 302 Found (wrong impl., On Linking Alternative Representations To Enable Discovery And Publishing).
There are a lot of sites that use "HTTP 301 Moved permanently" and "HTTP 303 See other" for content negotiation, but "HTTP 302 Found" (also called "HTTP 302 Moved temporarily") is the correct one. See also HTTP/1.1: Status Code Definitions.
Server responses
john@heimdal:~/tmp$ wget -S www.wikidata.org/entity/Q36661 --2015-06-25 10:59:13-- http://www.wikidata.org/entity/Q36661 Slår opp vertsnavn www.wikidata.org (www.wikidata.org) … 91.198.174.192, 2620:0:862:ed1a::1 Kobler til www.wikidata.org (www.wikidata.org)|91.198.174.192|:80 … tilkoblet. HTTP forespørsel sendt. Venter på svar … HTTP/1.1 301 TLS Redirect Server: Varnish Location: https://www.wikidata.org/entity/Q36661 Content-Length: 0 Accept-Ranges: bytes Date: Thu, 25 Jun 2015 08:59:14 GMT X-Varnish: 1438332533 Age: 0 Via: 1.1 varnish Connection: close X-Cache: cp3008 frontend miss (0) Set-Cookie: GeoIP=NO:11:Lillehammer:61.1333:10.5000:v4; Path=/; Domain=.wikidata.org Set-Cookie: WMF-Last-Access=25-Jun-2015;Path=/;HttpOnly;Expires=Mon, 27 Jul 2015 00:00:00 GMT Plassering: https://www.wikidata.org/entity/Q36661 [følgende] --2015-06-25 10:59:14-- https://www.wikidata.org/entity/Q36661 Kobler til www.wikidata.org (www.wikidata.org)|91.198.174.192|:443 … tilkoblet. HTTP forespørsel sendt. Venter på svar … HTTP/1.1 303 See Other Server: nginx/1.9.2 Date: Thu, 25 Jun 2015 08:59:14 GMT Content-Type: text/html; charset=iso-8859-1 Content-Length: 262 Connection: keep-alive X-Powered-By: HHVM/3.3.0-static Location: https://www.wikidata.org/wiki/Special:EntityData/Q36661 X-Varnish: 2371060664, 1854837344, 134795471 Via: 1.1 varnish, 1.1 varnish, 1.1 varnish Accept-Ranges: bytes Age: 0 X-Cache: cp1053 miss (0), cp3041 miss (0), cp3041 frontend miss (0) Strict-Transport-Security: max-age=1209600 X-Analytics: https=1;WMF-Last-Access=25-Jun-2015 Plassering: https://www.wikidata.org/wiki/Special:EntityData/Q36661 [følgende] --2015-06-25 10:59:15-- https://www.wikidata.org/wiki/Special:EntityData/Q36661 Bruker etablert tilkobling til www.wikidata.org:443 på nytt. HTTP forespørsel sendt. Venter på svar … HTTP/1.1 303 See Other <<== WRONG CODE Server: nginx/1.9.2 Date: Thu, 25 Jun 2015 08:59:15 GMT Content-Type: text/html; charset=utf-8 Transfer-Encoding: chunked Connection: keep-alive X-Powered-By: HHVM/3.6.1 X-Content-Type-Options: nosniff Vary: Accept-Encoding,X-Forwarded-Proto,Cookie Expires: Thu, 01 Jan 1970 00:00:00 GMT Location: https://www.wikidata.org/wiki/Special:EntityData/Q36661.json Last-Modified: Thu, 25 Jun 2015 08:59:15 GMT X-Varnish: 2436456097, 3983484236, 134797070 Via: 1.1 varnish, 1.1 varnish, 1.1 varnish Age: 0 X-Cache: cp1067 miss (0), cp3005 miss (0), cp3041 frontend miss (0) Strict-Transport-Security: max-age=1209600 Cache-Control: private, s-maxage=0, max-age=0, must-revalidate X-Analytics: https=1;WMF-Last-Access=25-Jun-2015 Plassering: https://www.wikidata.org/wiki/Special:EntityData/Q36661.json [følgende] --2015-06-25 10:59:15-- https://www.wikidata.org/wiki/Special:EntityData/Q36661.json Bruker etablert tilkobling til www.wikidata.org:443 på nytt. HTTP forespørsel sendt. Venter på svar … HTTP/1.1 200 OK Server: nginx/1.9.2 Date: Thu, 25 Jun 2015 08:59:15 GMT Content-Type: application/json; charset=UTF-8 Transfer-Encoding: chunked Connection: keep-alive X-Content-Type-Options: nosniff X-Powered-By: HHVM/3.6.1 Access-Control-Allow-Origin: * X-Frame-Options: DENY Vary: Accept-Encoding Last-Modified: Sat, 13 Jun 2015 14:37:25 GMT X-Varnish: 2033080611, 3336853391, 134798470 125619796 Via: 1.1 varnish, 1.1 varnish, 1.1 varnish Age: 1890 X-Cache: cp1066 miss (0), cp3008 miss (0), cp3041 frontend hit (2) Strict-Transport-Security: max-age=1209600 Cache-Control: private, s-maxage=0, max-age=0, must-revalidate X-Analytics: https=1;WMF-Last-Access=25-Jun-2015 Lengde: ikke angitt [application/json] Lagrer til: «Q36661»