$this->params['filter'] needs to be converted to an array.

### Summary

When using FilteredQuery, ```$request->toJSON()``` is missing filter data. In the example below, the output looks like this:

```{"filtered":{"query":{"match_all":{"boost":1}},"filter":{}}}```

The problem is that in [```FilteredQuery::toArray()```](https://github.com/polyfractal/sherlock/blob/master/src/Sherlock/components/queries/FilteredQuery.php#L32) we are not converting ```$this->params['filter']``` to an array, so when converting to JSON later, the Filter object is empty. Whoops!

This patch makes sure that it also gets converted to an array, returning the proper JSON of:

```{"filtered":{"query":{"match_all":{"boost":1}},"filter":{"term":{"foo":"bar","_cache":true}}}```

### Test

Run the following code in master:

```
$filter = Sherlock\Sherlock::filterBuilder()
							->Term()
								->field('foo')
								->term('bar

$query = Sherlock\Sherlock::queryBuilder()
						->matchAll();

$search = Sherlock\Sherlock::queryBuilder()
					->filteredQuery()
						->query($query)
						->filter($filter);
		
//Set the index, type and from/to parameters of the request.
$request->index("tweeter")
				->type("tweet")
				->query($search);

echo $request->toJSON();
```

Note that the request JSON is missing filter data. 

Next, run the same code in this branch, and note that the filter data is present.