correctly convert url to string when raising unsupported scheme error

client.rkt

@@ -166,7 +166,8 @@
          'get-document
          (string-append "unsupported scheme: ~a~n"
                         "  url: ~a~n")
-         (url-scheme destination) destination))
+         (url->string (url-scheme destination))
+         destination))
 
       (define-values (status meta body-port)
         (net:request destination))