Add missing $filter class property to FacetCollection

src/HAB/Solr/Facet/FacetCollection.php

@@ -50,6 +50,13 @@ class FacetCollection implements Countable, IteratorAggregate, ParameterProvider
      */
     private $facets;
 
+    /**
+     * Component state filter.
+     *
+     * @var StateFilter\StateFilterInterface
+     */
+    private $filter;
+
     /**
      * Constructor.
      *