Fixes Elasticsearch 6 tests.

wagtail/search/tests/test_elasticsearch6_backend.py

@@ -540,7 +540,8 @@ class TestElasticsearch6Mapping(TestCase):
                             'name': {'type': 'text', 'copy_to': '_all_text'},
                             'slug_filter': {'type': 'keyword'},
                         },
-                    }
+                    },
+                    'tags_filter': {'type': 'keyword'}
                 }
             }
         }
@@ -573,7 +574,8 @@ class TestElasticsearch6Mapping(TestCase):
             ],
             'publication_date_filter': datetime.date(1954, 7, 29),
             'number_of_pages_filter': 423,
-            'tags': []
+            'tags': [],
+            'tags_filter': []
         }
 
         self.assertDictEqual(document, expected_result)
@@ -642,7 +644,8 @@ class TestElasticsearch6MappingInheritance(TestCase):
                             'name': {'type': 'text', 'copy_to': '_all_text'},
                             'slug_filter': {'type': 'keyword'},
                         },
-                    }
+                    },
+                    'tags_filter': {'type': 'keyword'}
                 }
             }
         }
@@ -702,7 +705,8 @@ class TestElasticsearch6MappingInheritance(TestCase):
             ],
             'publication_date_filter': datetime.date(1954, 7, 29),
             'number_of_pages_filter': 423,
-            'tags': []
+            'tags': [],
+            'tags_filter': []
         }
 
         self.assertDictEqual(document, expected_result)