Merge branch 'master' of http://github.com/danielhasselrot/mongoengine into v0.4

Conflicts:
	mongoengine/fields.py

mongoengine/base.py

@@ -410,11 +410,18 @@ class BaseDocument(object):
             value = getattr(self, field_name, None)
             if value is not None:
                 data[field.db_field] = field.to_mongo(value)
+            else:
+                data[field.db_field] = None
         # Only add _cls and _types if allow_inheritance is not False
         if not (hasattr(self, '_meta') and
                 self._meta.get('allow_inheritance', True) == False):
             data['_cls'] = self._class_name
             data['_types'] = self._superclasses.keys() + [self._class_name]
+        try:
+            if not data['_id']:
+                del data['_id']
+        except KeyError:
+            pass
         return data
     
     @classmethod
@@ -446,7 +453,8 @@ class BaseDocument(object):
 
         for field_name, field in cls._fields.items():
             if field.db_field in data:
-                data[field_name] = field.to_python(data[field.db_field])
+                value = data[field.db_field]
+                data[field_name] = value if value is None else field.to_python(value)                 
 
         obj = cls(**data)
         obj._present_fields = present_fields