http-client: 'http-fetch' converts strings to URIs.

* guix/http-client.scm (http-fetch): Use 'string->uri' if URI is a
string.  Fixes a regression introduced in 8a5063f.

guix/http-client.scm

@@ -207,7 +207,9 @@ (define* (http-fetch uri #:key port (text? #f) (buffered? #t))
 unbuffered port, suitable for use in `filtered-port'.
 
 Raise an '&http-get-error' condition if downloading fails."
-  (let loop ((uri uri))
+  (let loop ((uri (if (string? uri)
+                      (string->uri uri)
+                      uri)))
     (let ((port (or port (open-connection-for-uri uri))))
       (unless buffered?
         (setvbuf port _IONBF))