fix send_zval_to_lua()

[tony2001]

Here's a code to test it:

$orig_callback = function() {
    return ['r'=>['user_id'=>10], 'e'=>''];
};
$Lua = new \lua();
$callback_code = 'return dump();';
$Lua->registerCallback("dump", $orig_callback);
var_dump($Lua->eval($callback_code));