DynamicDocument.reload doesn't work well

i encountered an issue when i was using DynamicDocument, here is the situation

In [1]: import mongoengine as models

In [2]: models.connect(port=11111)
Out[2]: MongoClient('localhost', 11111)

In [3]: class Foo(models.DynamicDocument):
    pass
   ...: 

In [4]: Foo.objects.delete()
Out[4]: 1

In [5]: foo = Foo.objects.create()

In [6]: foo._data
Out[6]: {'id': ObjectId('55910f4a53a70c17c0e2eb5e')}

In [7]: foo.update(bar=1)
Out[7]: 1

In [8]: foo._data
Out[8]: {'id': ObjectId('55910f4a53a70c17c0e2eb5e')}

In [9]: foo.reload()
Out[9]: <Foo: Foo object>

In [10]: foo._data
Out[10]: {'id': ObjectId('55910f4a53a70c17c0e2eb5e')}

In [11]: foo._dyn
foo._dynamic         foo._dynamic_fields  foo._dynamic_lock    

In [11]: foo._dynamic_fields
Out[11]: SON([])

In [12]: foo._fields_ordered
Out[12]: ('id',)

and it supposed to had bar reloaded, something like
{'bar': 1, 'id': ObjectId('55910f4a53a70c17c0e2eb5e')}

in https://github.com/MongoEngine/mongoengine/blob/master/mongoengine/document.py#L591 we can see that it loops over self._fields_ordered, which unfortunately DynamicDocument havn't

the simplest way i think is just loop over obj._data since obj is a complete instance

Start in mongoengine/document.py around line 591 and inspect how reload uses _fields_ordered for DynamicDocument instances. Compare that path with the complete instance data in _data, then verify that reloading a DynamicDocument restores dynamically added fields such as bar.