pay: fix #3613; string output must be quoted

`json_add_member` requires quotes for string types

Changelog-Fixed: `pay` would crash on expired waits with tried routes

plugins/pay.c

@@ -262,7 +262,7 @@ static struct command_result *waitsendpay_expired(struct command *cmd,
 	for (size_t i = 0; i < tal_count(pc->ps->attempts); i++) {
 		json_object_start(data, NULL);
 		if (pc->ps->attempts[i].route)
-			json_add_member(data, "route", false, "%s",
+			json_add_member(data, "route", true, "%s",
 					 pc->ps->attempts[i].route);
 		json_out_add_splice(data->jout, "failure",
 				    pc->ps->attempts[i].failure);