Send shutdown_reply on the control socket for protocol 5.4

This is the only change in 5.4, in previous versions it was sent on the shell
socket.

Author: JamesWrigley

src/handlers.jl

@@ -237,7 +237,9 @@ function shutdown_request(socket, kernel, msg)
     global _shutting_down[] = true
     @async Base.throwto(kernel.requests_task[], InterruptException())
 
-    send_ipython(kernel.requests[], kernel,
+    # In protocol 5.4 the shutdown reply moved to the control socket
+    shutdown_socket = VersionNumber(msg) >= v"5.4" ? kernel.control[] : kernel.requests[]
+    send_ipython(shutdown_socket, kernel,
                  msg_reply(msg, "shutdown_reply", msg.content))
     sleep(0.1) # short delay (like in ipykernel), to hopefully ensure shutdown_reply is sent
     kernel.shutdown()

src/msg.jl

@@ -14,6 +14,8 @@ msg_header(m::Msg, msg_type::String) = Dict("msg_id" => uuid4(),
                                             "msg_type" => msg_type,
                                             "version" => "5.4")
 
+Base.VersionNumber(m::Msg) = VersionNumber(m.header["version"])
+
 # PUB/broadcast messages use the msg_type as the ident, except for
 # stream messages which use the stream name (e.g. "stdout").
 # [According to minrk, "this isn't well defined, or even really part