mod_pubsub: Fix merging of default node options

Use any option specified via 'default_node_config' by default, and take
the remaining defaults from the node plugin handling the request.  This
is the documented behavior.

Before this change, the code applied the plugin defaults only if no
'default_node_config' existed at all.  And even this logic didn't work
as intended, since 'default_node_config' yielded an empty list in case
it wasn't configured, which resulted in plugin defaults never being
applied.

src/mod_pubsub.erl

@@ -3367,9 +3367,10 @@ get_option(Options, Var, Def) ->
 -spec node_options(host(), binary()) -> [{atom(), any()}].
 node_options(Host, Type) ->
     DefaultOpts = node_plugin_options(Host, Type),
+    ConfigOpts = config(Host, default_node_config),
     case lists:member(Type, config(Host, plugins)) of
 	true ->
-            config(Host, default_node_config, DefaultOpts);
+            merge_config([ConfigOpts, DefaultOpts]);
 	false -> DefaultOpts
     end.