Fix image loading not working after a while

http-easy connections are pooled, and streaming responses does not
automatically close the response (they are closed automatically in
other situations). After 128 (the default) opened connections to fetch
images, the pool ran out of connections, and future attempts would
just time out waiting for a connection.

Fixed by manually closing the connection at the correct time.

src/page-proxy.rkt

@@ -24,6 +24,7 @@
              #:code (easy:response-status-code dest-r)
              #:mime-type (easy:response-headers-ref dest-r 'content-type)
              (λ (out)
-               (copy-port (easy:response-output dest-r) out)))))
+               (copy-port (easy:response-output dest-r) out)
+               (easy:response-close! dest-r)))))
          (next-dispatcher))]
     [#f (next-dispatcher)]))