fix-#746: Stop ensure_indexes running on a secondaries unless connection is through mongos

mongoengine/document.py

@@ -594,7 +594,9 @@ class Document(BaseDocument):
         index_cls = cls._meta.get('index_cls', True)
 
         collection = cls._get_collection()
-        if collection.read_preference > 1:
+        # 746: when connection is via mongos, the read preference is not necessarily an indication that
+        # this code runs on a secondary
+        if not collection.is_mongos and collection.read_preference > 1:
             return
 
         # determine if an index which we are creating includes