[Fix] Add `**kwargs` to `to_mongo()`

[malthejorgensen]

Since mongoengine@30fdd3e184165b722cd470fdbd470634374bff77, the to_mongo()-method is passed a fields keyword argument when saving.

The current extras-mongoengine master doesn't work with the current mongoengine master (2016-04-28), as the fields defined in this package is missing the handling of **kwargs.