Create a dbus proxy object will memory leak If the return type of method is HashTable which is contains another HashTable.
Following simple code reproduces the issues:

Vala code:
>    [DBus (name = "org.freedesktop.DBus.ObjectManager")]
>    public interface DBusObjectManager : Object
>    {
>        public abstract HashTable<ObjectPath, HashTable<string, HashTable<string, Variant>>> get_managed_objects();
>    }

C code:
>    static GHashTable* dbus_object_manager_proxy_get_managed_objects (DBusObjectManager* self) {
>    …
>    g_variant_iter_init (&_tmp2_, _tmp0_);
>    _tmp1_ = g_hash_table_new_full (g_direct_hash, g_direct_equal, NULL, NULL);
>    g_variant_iter_init (&_tmp2_, _tmp0_);
>    while (g_variant_iter_loop (&_tmp2_, "{?*}", &_tmp3_, &_tmp4_)) {
>    	GHashTable* _tmp5_;
>    	GVariantIter _tmp6_;
>    	GVariant* _tmp7_;
>    	GVariant* _tmp8_;
>    	_tmp5_ = g_hash_table_new_full (g_str_hash, g_str_equal, g_free, NULL);
>    	g_variant_iter_init (&_tmp6_, _tmp4_);
>    	while (g_variant_iter_loop (&_tmp6_, "{?*}", &_tmp7_, &_tmp8_)) {
>    		GHashTable* _tmp9_;
>    		GVariantIter _tmp10_;
>    		GVariant* _tmp11_;
>    		GVariant* _tmp12_;
>    		_tmp9_ = g_hash_table_new_full (g_str_hash, g_str_equal, g_free, (GDestroyNotify) g_variant_unref);
>    …

The generated C code did not specify a value destroy function in g_hash_table_new_full if the original HashTable contains another HashTable.

Observing the function of deserialize_hash_table() in codegen/valagvariantmodule.vala,
Create the key/value destroy function for HashTable that only take care of String & Variant type,
would it be possible to add expanded support for other type?