Fix socket leak on redirected `*-pure-port' calls.

A port over which a redirection is served to get-pure-port is not closed
if `get-pure-port' follows the redirection.  This leaks file descriptors
on the host machine.

collects/net/url.rkt

@@ -254,6 +254,7 @@
       (regexp-match #rx"^HTTP/[0-9]+[.][0-9]+ 3[0-9][0-9]" status))
     (cond
       [(and redirection-status-line? new-url (not (zero? redirections))) 
+       (close-input-port ip)
        (redirection-loop (- redirections 1) new-url)]
       [else
        (define-values (in-pipe out-pipe) (make-pipe))