plugin: Unwrap the rpc_command payload

We were nesting like the following:

```json
{"params": {
  "rpc_command": {
    "rpc_command": {
    }
  }
}
```

This is really excessive, so we unwrap once, and now have the following:

```json
{"params": {
  "rpc_command": {
  }
}
```

Still more wrapping than necessary (the method is repeated in the `params`
object), but it's getting closer.

Suggested-by: @fiatjaf
Signed-off-by: Christian Decker <@cdecker>

lightningd/jsonrpc.c

@@ -611,8 +611,21 @@ struct rpc_command_hook_payload {
 static void rpc_command_hook_serialize(struct rpc_command_hook_payload *p,
                                        struct json_stream *s)
 {
+	const jsmntok_t *tok;
+	size_t i;
+	char *key;
 	json_object_start(s, "rpc_command");
+
+#ifdef COMPAT_V081
 	json_add_tok(s, "rpc_command", p->request, p->buffer);
+#endif
+
+	json_for_each_obj(i, tok, p->request) {
+		key = tal_strndup(NULL, p->buffer + tok->start,
+				  tok->end - tok->start);
+		json_add_tok(s, key, tok + 1, p->buffer);
+		tal_free(key);
+	}
 	json_object_end(s);
 }
 

tests/plugins/rpc_command.py

@@ -9,7 +9,7 @@
 
 @plugin.hook("rpc_command")
 def on_rpc_command(plugin, rpc_command, **kwargs):
-    request = rpc_command["rpc_command"]
+    request = rpc_command
     if request["method"] == "invoice":
         # Replace part of this command
         request["params"]["description"] = "A plugin modified this description"