Allow to reset ordering by calling order_by() with no arguments

DIFFERENCES.md

@@ -21,6 +21,7 @@ Differences between Mongomallard and Mongoengine
 * `_get_changed_fields()` / `_changed_fields` returns a set
 * Simplified `EmailField` email regex to be more compatible
 * Assigning invalid types (e.g. an invalid string to `IntField`) raises immediately a `ValueError`
+* `order_by()` without an argument resets the ordering (no ordering will be applied)
 
 Untested / not implemented yet:
 -----

mongoengine/queryset/queryset.py

@@ -53,7 +53,7 @@ class QuerySet(object):
         self._initial_query = {}
         self._where_clause = None
         self._loaded_fields = QueryFieldList()
-        self._ordering = []
+        self._ordering = None
         self._snapshot = False
         self._timeout = True
         self._class_check = True
@@ -1211,7 +1211,7 @@ class QuerySet(object):
             if self._ordering:
                 # Apply query ordering
                 self._cursor_obj.sort(self._ordering)
-            elif self._document._meta['ordering']:
+            elif self._ordering == None and self._document._meta['ordering']:
                 # Otherwise, apply the ordering from the document model
                 order = self._get_order_by(self._document._meta['ordering'])
                 self._cursor_obj.sort(order)