Tuples converted to MATLAB structs

[jebej]

It seems that tuples are currently being converted to structs, which is wrong. I think it makes more sense to convert tuples to cell arrays. Thoughts?

julia> jvalue(mxarray((2.5,2.6)))
Dict{String,Any} with 1 entry:
  "2.5" => 2.6

As an aside, the reason this happens is because of a default fallback: https://github.com/JuliaInterop/MATLAB.jl/blob/6e859c1a126b345662b0282a70c9c0c89d9a2762/src/mxarray.jl#L474

Maybe this should be changed, it doesn't seem like a great choice.