Allow setting the read concern to None

Author: abarto

mongoengine/queryset/base.py

@@ -1227,7 +1227,7 @@ def read_concern(self, read_concern):
             raise TypeError("%r is not a valid read concern." % (read_concern,))
 
         queryset = self.clone()
-        queryset._read_concern = ReadConcern(**read_concern)
+        queryset._read_concern = ReadConcern(**read_concern) if read_concern is not None else None
         queryset._cursor_obj = None  # we need to re-create the cursor object whenever we apply read_concern
         return queryset