net/http-client: restore use of abandon

The difference between `abandon` and `close-output-port` seems to
matter for downloading PLaneT packages on some networks. Using
`abandon` does not seem to create any problems now that an extra
"\r\n" is not incorrecty included in PUT/POST requests.

racket/collects/net/http-client.rkt

@@ -90,7 +90,7 @@
 (define (http-conn-abandon! hc)
   (match-define (http-conn host port port-usual? to from abandon) hc)
   (when to
-    (close-output-port to)
+    (abandon to)
     (set-http-conn-to! hc #f)))
 
 (define (http-conn-send! hc url-bs