dump-only fields are passed when unknown = INCLUDE

When unknown=INCLUDE, dump-only fields are passed as unknown (and not validated).

This is due to https://github.com/marshmallow-code/marshmallow/pull/865 and is a side-effect of being considered unknown, but is this really what we want?

Modified test:

    def test_dump_only_fields_considered_unknown(self):
        class MySchema(Schema):
            foo = fields.Field(dump_only=True)

        with pytest.raises(ValidationError) as excinfo:
            MySchema().load({'foo': 42})
        err = excinfo.value
        assert 'foo' in err.messages
        assert err.messages['foo'] == ['Unknown field.']

        # Test with unknown=INCLUDE
        data = MySchema(unknown=INCLUDE).load({'foo': 42})
        assert 'foo' not in data  # fail

Excluding them seems inconsistent (why exclude a dump-only field and not a real unknown field if dump-only are treated as unknown?) but less risky.

I’m not saying this is a bug, but I just got surprised by it and it is not explicitly tested, so I figured I’d ask.

[sloria]

I also find the current behavior to be the most intuitive. Unless there are any strong objections, I’ll plan on merging #981 within the next few days.

[deckar01]

What if dump_only accepted RAISE/EXCLUDE values and could be set to EXCLUDE to explicitly opt in to the silently ignore behavior?