vNext - DUs with a single Case but multiple arguments aren't encoded correctly

[MangelMaxime]

type SeveralArgumentDus = SeveralArgs of int * {| Name : string; Age : int |}

let value = SeveralArgs (10, {| Name = "maxime"; Age = 28 |})
let json = Encode.Auto.toString(4, value)
printfn "%A" json // Print: 10