[BUG] - toml is not correctly using custom serialize impl

[oknozor]

Seems related to #603, #636

When serializing a struct containing Option with a custom serializer, we get an UnsupportedType(None) error.
Note that this work as expected using toml < 0.5 and panic starting from 0.6.

Here is a minimal reproducer:

use std::fmt;
use std::fmt::Formatter;
use serde::{Serialize, Serializer};

struct Person {
    name: String,
    age: Option<u16>
}

impl Serialize for Person {
    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
        where
            S: Serializer,
    {
        serializer.serialize_str(&self.to_string())
    }
}

impl fmt::Display for Person {
    fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
        write!(f, "{}", self.name)
    }
}

fn main() {
    // Panics on toml = ^0.6
    // Works with toml = 0.5
    toml::to_string(&Person {
        name: "tom".to_string(),
        age: None,
    }).unwrap();
}

[epage]

This is serializing a string as a top-level TOML document which is invalid TOML syntax. If you want to deserialize a TOML value, you can use ValueSerailizer. As this is working as expected, I'm closing this, If there is a reason for us to re-evaluate this, let us know!