Let incomplete closure tests pass

TALES path evaluation logic was not enough to pass closure tests.
They can be passed if using this modification.
The intent is to only support closures specifically. Otherwise I'm worried it could lead to ambiguous code, e.g. 

```php
$tpl->var = ["class","method"];
```

```xml
<x tal:content="var/0"/>
```

does it read `0` property from data returned by `class::method()` call or just return string `"class"` from the first element of the array?

Too much weird stuff is callable in PHP.