Adding magic __call()

I'm using Elastica with Symfony and when I try to call for a key that has ``null`` as value the Symfony throws:
```
Method "keyName" for object "Elastica\Result" does not exist in MyBundle:FolderName:myTemplate.html.twig at line X
```

Inside twig template I call like this:
```twig
{{ elasticaResultVariable.keyName }}
```

Adding this magic ``__call`` it works.

Maybe there is something better to do, so you can cancel this PR.