Unexpected behaviour when reference to array element is created

Example:

``` lua
function myfunc(data)
end
```

``` php
$data = [
     'key1' => 'value1',
];
$link = &$data['key1'];
$tarantool->call('myfunc', [$data]);
```

Problem:
Inside lua-function 'myfunc' i see that data['key1'] is null (cdata<void *>: NULL).

Expected behaviour:
data['key1'] must be 'value1' in lua-function.
