redo fix for ListField loses use_db_field when serializing #1217

The new fix reverted the change on BaseField to_mongo so that it will not force
new field class to add kwargs to to_mongo function. The new derived field class
to_mongo can support use_db_field and fields parameters as needed.
Basically all field classes derived from ComplexBaseField support those parameters.

tests/fields/fields.py

@@ -3527,7 +3527,7 @@ class FieldTest(unittest.TestCase):
             def __init__(self, **kwargs):
                 super(EnumField, self).__init__(**kwargs)
 
-            def to_mongo(self, value, **kwargs):
+            def to_mongo(self, value):
                 return value
 
             def to_python(self, value):