suggestion: _message_fields_schema black list instead of white list.

[matantsu]

it would be nice if I could provide a list of properties to NOT include in the protorpc message. kind of the opposite of _message_fields_schema.

[dhermes]

Similarly, additional fields would also be nice (i.e. non-default, but valid).

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Simplest way to get a blacklist would be to use:

fields = MyModel._DefaultFields()
for field in black_list:
    fields.remove(field)

and then use fields as _message_fields_schema.