build http parts
roles = [ "admin" ] |> Set.fromList
comments = [ "good", "great" ]
[ ( "name", "John" |> Part.string )
, ( "age", 30 |> Part.int )
, ( "file", file |> Part.file )
, ( "roles", roles |> Part.set Part.string )
, ( "comments", comments |> Part.list Part.string )
] |> Part.object
part value