Parsing subchild document list return an empty list. This fix it

My model looks like : 

    class Datas(EmbeddedDocument):
        title = StringField()
        dataset = ListField(DynamicField())
    class Widget(Document):
        datas = EmbeddedDocumentField(Datas)

Actually, the serializer returns empty datasets. 
     
    widget  {
        datas : {
           title:"dataset",
            dataset : [] 
    }
    }

This pull request fix it