added instanceof check to cast Twig_Markup objects to string

using the following codeblock in a twig template leads to the variable ```my_js_var``` beeing a ```Twig_Markup``` object.
```
{% set my_js_var %}
    alert('my multiline js');
    alert('another alert');
{% endset %}
```
adding this with ```add_assets``` resulting in an empty ```$this->packages```
```
{{ add_assets(my_js_var, 'inline_js') }}
```

with this patch the ```Twig_Markup``` will be cast to ```string```