Added support for additional_query_string in mlt

tests/xapian_backend.py

@@ -262,6 +262,10 @@ class XapianSearchBackendTestCase(TestCase):
         results = self.sb.more_like_this(self.sample_objs[0])
         self.assertEqual(results['hits'], 2)
         self.assertEqual([result.pk for result in results['results']], [3, 2])
+
+        results = self.sb.more_like_this(self.sample_objs[0], additional_query_string='david3')
+        self.assertEqual(results['hits'], 1)
+        self.assertEqual([result.pk for result in results['results']], [3])
     
     def test_document_count(self):
         self.sb.update(self.msi, self.sample_objs)

xapian_backend.py

@@ -410,8 +410,11 @@ class SearchBackend(BaseSearchBackend):
             xapian.Query.OP_AND_NOT, [query, self.get_identifier(model_instance)]
         )
         if additional_query_string:
+            additional_query, __unused__ = self._query(
+                database, additional_query_string
+            )
             query = xapian.Query(
-                xapian.Query.OP_AND, [query, self._query(database, additional_query_string)]
+                xapian.Query.OP_AND, query, additional_query
             )
         enquire.set_query(query)