Mixing types does not work

Hi! There is a very confusing error thrown in some cases with mixed types.

A minimal example:

    df = DataFrame(a=1:1)
    df |>
        @mapmany([(c=Inf,), (c=1,)], (; _..., b=__)) |>
        @groupby({_.b}) |>
        @map((; key(_)..., a=_.a))

throws type NamedTuple has no field a on the last line with @map. This is due to c having type Float in one row, and Int in another. When all values have the same type, the same code works:

    df = DataFrame(a=1:1)
    df |>
        @mapmany([(c=0,), (c=1,)], (; _..., b=__)) |>  # changed Inf to 0
        @groupby({_.b}) |>
        @map((; key(_)..., a=_.a))

Julia 1.2.0, Query v0.12.1.

Reproduce the minimal example with Julia 1.2.0 and Query v0.12.1, then compare it with the version where Inf is replaced by 0. Trace the interaction among @mapmany, @groupby, and the final @map when mixed types are present. Done means the mixed-type example no longer throws the NamedTuple field error and retains access to a.