validate=False seemingly has no effect in to_dict() for encoding channel classes

[joelostblom]

While alt.Chart().to_dict(validate=False) bypasses validation as expected, it seems like the same is not true for the encoding classes. For example, alt.Color('test').to_dict(validate=False) raises ValueError: test encoding field is specified without a type

[binste]

I agree that it's more intuitive if we don't raise the error if validate=False and so that it's a bug. One minor addition: validate=False still disables the jsonschema validation for channels but we don't even get that far as there is this ValueError which is raised already in FieldChannelMixin.to_dict.