Compatibility of schemas with nested types

[poncito]

Hi,

Here is a minimal example of the issue I've encountered.

 struct A
    x::Int
end

struct B
    a::A
end

v = [B(A(i)) for i =1:3]

io = IOBuffer()
Arrow.write(io, v; file=false)
seekstart(io)
Arrow.append(io, v) # throws

I don't know if this is really necessary, or if I'm not using this library properly, but this issue makes it difficult to append to arrow files with nested types.

Since I've only added more cases where the call to append can succeed, I do not think that this creates retro-compatibility issues.

Thanks for the review!

[poncito]

thanks Ben!