facet: broken shorthand string syntax for encodings

[tbenst]

This matters as Layer cannot be used with row or column, so must use facet:

# This works
dataset("cars.json") |> @vlplot(
    facet={row={field="Origin", type="nominal"}},
    spec={
        :bar,
        x={"Horsepower:q", bin={maxbins=15}},
        y="count()"

    }
)

# this doesn't
dataset("cars.json") |> @vlplot(
    facet={row="Origin:n"},
    spec={
        :bar,
        x={"Horsepower:q", bin={maxbins=15}},
        y="count()"

    }
)