Improve the content negotiation handling in general

Previously, the routing layer handled the content negotiation, and the Accept
header was ignored. Now, the extension if one is provided in the URL is still
used, and more widely than before, but the Accept header is also taken in to
account.

This all now happens before the routing decisions are made, so the routing is
now pretty much extension independant (with the exception of the
/gnu/store/... routes).

guix-data-service/web/server.scm

@@ -18,6 +18,7 @@
 
 (define-module (guix-data-service web server)
   #:use-module (srfi srfi-1)
+  #:use-module (srfi srfi-11)
   #:use-module (web http)
   #:use-module (web request)
   #:use-module (web uri)
@@ -30,9 +31,14 @@
 (define (run-controller controller request body)
   (with-postgresql-connection
    (lambda (conn)
-     ((controller request body conn)
-      (cons (request-method request)
-            (request-path-components request))))))
+     (let-values (((request-components mime-types)
+                   (request->path-components-and-mime-type request)))
+       (controller request
+                   (cons (request-method request)
+                         request-components)
+                   mime-types
+                   body
+                   conn)))))
 
 (define (handler request body controller)
   (format #t "~a ~a\n"