http-client: Accept #:open-connection in http-fetch.

So that an alternative procedure can be passed in, perhaps to perform
connection caching.

* guix/http-client.scm (http-fetch): Add an #:open-connection keyword
argument.

guix/http-client.scm

@@ -75,6 +75,7 @@ (define-condition-type &http-get-error &error
 
 
 (define* (http-fetch uri #:key port (text? #f) (buffered? #t)
+                     (open-connection guix:open-connection-for-uri)
                      (keep-alive? #f)
                      (verify-certificate? #t)
                      (headers '((user-agent . "GNU Guile")))
@@ -97,10 +98,10 @@ (define* (http-fetch uri #:key port (text? #f) (buffered? #t)
   (let loop ((uri (if (string? uri)
                       (string->uri uri)
                       uri)))
-    (let ((port (or port (guix:open-connection-for-uri uri
-                                                       #:verify-certificate?
-                                                       verify-certificate?
-                                                       #:timeout timeout)))
+    (let ((port (or port (open-connection uri
+                                          #:verify-certificate?
+                                          verify-certificate?
+                                          #:timeout timeout)))
           (headers (match (uri-userinfo uri)
                      ((? string? str)
                       (cons (cons 'Authorization