kernels: 'reply-html' replies to the message sender.

* jupyter/kernels.scm (reply-html): Pass #:recipient to 'send-message'.

jupyter/kernels.scm

@@ -446,7 +446,8 @@ identity (bytevector)."
                 (send-message kernel
                               (reply message type
                                      (scm->json-string content))
-                              #:kernel-socket socket))))
+                              #:kernel-socket socket
+                              #:recipient (message-sender message)))))
 
     (send kernel-iopub
           "execute_input" `(("code"            . ,code)