serde::to_vec(rmpv::Value) works unexpected

Hi!
Looks like rmp_serde::to_vec works not properly for rmpv::Value with map inside it. I'm expected that result is encoded message pack vector, but result is a map. For example:

```rust
#[test]
fn serialize_value_as_map() {
let value = rmpv::Value::Map(vec![
(rmpv::Value::from("a"), rmpv::Value::from(1)),
(rmpv::Value::from("b"), rmpv::Value::from(2)),
]);
let encoded = rmp_serde::to_vec(&value).unwrap();

assert_eq!(vec![130, 161, 97, 1, 161, 98, 2], encoded);
}
```