Cannot append! KeyedArray to StructArray

[aplavin]

Found a bug in the append! function of StructArrays:

julia> using AxisKeys, StructArrays

# OK:
julia> append!(StructArray(a=[1,2]), [(a=3,),(a=4,)])
4-element StructArray(::Vector{Int64}) with eltype NamedTuple{(:a,), Tuple{Int64}}:
...

# error:
julia> append!(StructArray(a=[1,2]), KeyedArray([(a=3,),(a=4,)], 1:2))
ERROR: type NamedTuple has no field a

[aplavin]

I guess related to #270 (comment). Surely pushing/appending arrays shouldn't depend on Tables, right? This risks inconsistencies between the regular array and Table interfaces, and here we see a real-life example.

TBH, I didn't encounter it in the wild, but specifically crafted after looking how tables are incorporated in StructArrays. But I think this surely is a bug.