test: add untagged de tests

[Fishrock123]

I was unconvinced that serde_yaml's deserializer was handling untagged enums correctly, but it appears that it is.

[polarathene]

For reference, there's a known failure that affects untagged enums: #165

---

The reproduction example (original Rust Playground):

use std::collections::BTreeMap;
use serde_yaml;
use serde::Deserialize;

fn main() {
    let input = r#"
items:
    bar: 42
    baz: 123
    200: 321
"#;

    // Deserializing into `Items` will work, but not `Wrapper` as an untagged enum:
    let wrapped: Wrapper = serde_yaml::from_str(input).expect("Couldn't deserialize");
    println!("{:#?}", wrapped);
}

#[derive(Debug, Deserialize, PartialEq)]
#[serde(untagged)]
enum Wrapper {
  Item(Items),
}

#[derive(Debug, Deserialize, PartialEq)]
struct Items {
  items: BTreeMap<String, usize>,
}

Fails with:

Couldn't deserialize: Message("data did not match any variant of untagged enum Wrapper", None)

EDIT: The above playground reproduction uses an 0.8.x release. The same failure occurs with 0.9.x too however.

It's due to number key 200 not being quote wrapped. Seems to be an issue with converting it to the desired String key in the Items struct.