fixes toArray function of Elastica\Filter\Bool

This branch fixes the toArray function of Elastica\Filter\Bool to allow children Elastica\Filter\Bool filters in the should, must and must_not arrays. This functionality is shown in [the ElasticSearch documentation](http://www.elasticsearch.org/guide/en/elasticsearch/guide/current/combining-filters.html).

Additionally a test class is provided for this filter.

@ruflin: Can you please check this fix out? It would be great if this fix could end up in the master. Thanks!