Fixed headers support for inbound leafnode connection

The server that solicits a LeafNode connection does not send an
INFO, so the accepting side had no way to know if the remote
supports headers or not. The solicit side will now send the headers
support capability in the CONNECT protocol so that the receiving
side can mark the inbound connection with headers support based
on that and its own support for headers.

Resolves #1781

Signed-off-by: Ivan Kozlovic <ivan@synadia.com>

test/leafnode_test.go

@@ -4240,3 +4240,61 @@ func TestLeafNodeStreamAndShadowSubs(t *testing.T) {
 		t.Fatalf("Did not receive message: %v", err)
 	}
 }
+
+func TestLeafnodeHeaders(t *testing.T) {
+	srv, opts := runLeafServer()
+	defer srv.Shutdown()
+	leaf, _ := runSolicitLeafServer(opts)
+	defer leaf.Shutdown()
+
+	snc, err := nats.Connect(srv.ClientURL())
+	if err != nil {
+		t.Fatalf(err.Error())
+	}
+	defer snc.Close()
+	ssub, err := snc.SubscribeSync("test")
+	if err != nil {
+		t.Fatalf("subscribe failed: %s", err)
+	}
+
+	lnc, err := nats.Connect(leaf.ClientURL())
+	if err != nil {
+		t.Fatalf(err.Error())
+	}
+	defer lnc.Close()
+	lsub, err := lnc.SubscribeSync("test")
+	if err != nil {
+		t.Fatalf("subscribe failed: %s", err)
+	}
+	lnc.Flush()
+
+	checkLeafNodeConnected(t, srv)
+	checkLeafNodeConnected(t, leaf)
+	checkSubInterest(t, srv, "$G", "test", time.Second)
+
+	msg := nats.NewMsg("test")
+	msg.Header.Add("Test", "Header")
+	if len(msg.Header) == 0 {
+		t.Fatalf("msg header is empty")
+	}
+	err = snc.PublishMsg(msg)
+	if err != nil {
+		t.Fatalf(err.Error())
+	}
+
+	smsg, err := ssub.NextMsg(time.Second)
+	if err != nil {
+		t.Fatalf("next failed: %s", err)
+	}
+	if len(smsg.Header) == 0 {
+		t.Fatalf("server msgs header is empty")
+	}
+
+	lmsg, err := lsub.NextMsg(time.Second)
+	if err != nil {
+		t.Fatalf("next failed: %s", err)
+	}
+	if len(lmsg.Header) == 0 {
+		t.Fatalf("leaf msg header is empty")
+	}
+}