NamedTuple{...,Union{...}} values are serializable but inaccessible once deserialized

[jrevels]

similarly flavored problem to #75:

julia> write_table(tbl) = (io = IOBuffer(); Arrow.write(io, tbl); seekstart(io); Arrow.Table(io))
write_table (generic function with 1 method)

julia> struct Foo
           a::Union{Int,String}
       end

julia> tbl = write_table((foos=[Foo(1), Foo("x")],))
Arrow.Table: (foos = Foo[Foo(1), Foo("x")],)

julia> tbl.foos[1] # works fine
Foo(1)

julia> const FooEmulation = NamedTuple{(:a,),Tuple{Union{Int,String}}}
NamedTuple{(:a,),Tuple{Union{Int64, String}}}

julia> tbl2 = write_table((foos=FooEmulation[(a=1,),(a="x",)],));

julia> tbl2.foos[1] # uh oh
ERROR: MethodError: Cannot `convert` an object of type Int64 to an object of type Arrow.UnionT{Arrow.Flatbuf.UnionModeModule.Dense,nothing,Tuple{Union{Missing, Int64},String}}
Closest candidates are:
  convert(::Type{T}, ::T) where T at essentials.jl:171
Stacktrace:
 [1] convert(::Type{Tuple{Arrow.UnionT{Arrow.Flatbuf.UnionModeModule.Dense,nothing,Tuple{Union{Missing, Int64},String}}}}, ::Tuple{Int64}) at ./essentials.jl:310
 [2] Tuple{Arrow.UnionT{Arrow.Flatbuf.UnionModeModule.Dense,nothing,Tuple{Union{Missing, Int64},String}}}(::Tuple{Int64}) at ./tuple.jl:225
 [3] NamedTuple{(:a,),Tuple{Arrow.UnionT{Arrow.Flatbuf.UnionModeModule.Dense,nothing,Tuple{Union{Missing, Int64},String}}}}(::Tuple{Int64}) at ./namedtuple.jl:90
 [4] getindex(::Arrow.Struct{NamedTuple{(:a,),Tuple{Arrow.UnionT{Arrow.Flatbuf.UnionModeModule.Dense,nothing,Tuple{Union{Missing, Int64},String}}}},Tuple{Arrow.DenseUnion{Arrow.UnionT{Arrow.Flatbuf.UnionModeModule.Dense,nothing,Tuple{Union{Missing, Int64},String}},Tuple{Arrow.Primitive{Union{Missing, Int64},Array{Int64,1}},Arrow.List{String,Int32,Array{UInt8,1}}}}}}, ::Int64) at /Users/jarrettrevels/.julia/dev/Arrow/src/arraytypes/struct.jl:38
 [5] top-level scope at REPL[65]:1