flatten does't respect the rpm_serde::to_vec naming removal

[danielmeza]

Whe using MessagePack with flatten struct and the pm_serde::to_vec the serializer produce properties names for flatten structs.

It should not produce these names since pm_serde::to_vec is supposed to generate array separated properties (by position)

#[derive(Serializer)]
pub struct BaseStruct
{
    pub a: int;
    pub b: int;
}

#[derive(Serializer)]
pub struct RootStruct
{
   #[serde(flatten)]
   pub base: BaseStruct;
   pub c: int;
}

Current MessagePack binary => JSON output

[
  "base": {
    "a": 1, 
    "b": 2
  },
  3
]

Expected output:

[1,2,3]

Based on this issue its a flatten limitation that needs attention:

3Hren/msgpack-rust#296