colmetadata does not read custom metadata with multiple writes

[JakeZw]

This problem is described in more detail in discourse.

Specifically the following MWE works when the for loop executes once, but the colmetadata gives nothing when the for loop executes more than once.

`using Arrow
using Tables

go to runtest.jl in Arrow package for hints on how to use metadata and colmetadata

cd(@DIR)
filename = "test10.arrow"
meta = ["file" => "test.arrow", "when" => "now", "why" => "gain experience"]
metacol1 = ["id" => "chan1", "comment" => "Chan 1 comment"]
metacol2 = ["id" => "chan2", "comment" => "Chan 2 comment"]
writer = open(Arrow.Writer, filename, metadata=meta, colmetadata = Dict(:Column1 => metacol1, :Column2 => metacol2))

for i in 1:1
result = rand(2, 2) .+ i
Arrow.write(writer, Tables.table(result)) #, header=[:a,:b])
end
close(writer)

tbl = Arrow.Table(filename)

tblmeta = Arrow.getmetadata(tbl)
tblmeta
tblmeta["why"]
tblmeta["when"]

tblmetachan1 = Arrow.getmetadata(tbl.Column1)
tblmetachan1
tblmetachan1 == nothing
`

For once through the for loop we get
julia> tblmetachan1 = Arrow.getmetadata(tbl.Column1) Base.ImmutableDict{String, String} with 2 entries: "comment" => "Chan 1 comment" "id" => "chan1"

When we go through the for loop twice (or more) we get
`julia> tblmetachan1 = Arrow.getmetadata(tbl.Column1)

julia> tblmetachan1

julia> tblmetachan1 == nothing
true`