Fix pivot facet response parsing

broken since 6c8ed118

library/Solarium/QueryType/Select/ResponseParser/Component/FacetSet.php

@@ -247,7 +247,7 @@ class FacetSet extends ResponseParserAbstract implements ComponentParserInterfac
      */
     protected function facetPivot($query, $facet, $data)
     {
-        $key = implode(',', $facet->getFields());
+        $key = $facet->getKey();
         if (!isset($data['facet_counts']['facet_pivot'][$key])) {
             return null;
         }

tests/Solarium/Tests/QueryType/Select/ResponseParser/Component/FacetSetTest.php

@@ -97,7 +97,7 @@ class FacetSetTest extends \PHPUnit_Framework_TestCase
                     )
                 ),
                 'facet_pivot' => array(
-                    'cat,price' => array(
+                    'keyE' => array(
                         array(
                             'field' => 'cat',
                             'value' => 'abc',