I'm reading some yaml, editing and then saving it again after doing to_string.
For some reason I'm not understanding, some String values which only contain numbers end up not having the double quotes after to_string.
I'm feeding this yaml to something that expects Strings, not numbers, and I'm expecting serde_yaml to respect whichever type the value was.
Here's a minimal reproduceable test, which passes if I use serde_yaml 0.8.26 but does not pass if I use 0.9:

https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=6de99a49da249819a76e5fcd25342ec3

extern crate serde;
extern crate serde_yaml;


#[test]
fn can_serialize_with_quotes() {
    use serde_yaml::Mapping;
    let original_config = r#"---
configuration:
  agent: "007"
"#;
    let config: Mapping = serde_yaml::from_str(original_config).expect("should parse yaml");
    let config = serde_yaml::to_string(&config).expect("should serialize");
    assert_eq!(config, original_config);
}

basically this input:

configuration:
  agent: "007"

ends up as:

configuration:
  agent: 007

(there is also another difference between 0.8 and 0.9 regarding having or not having ---\n at the beginning of the String, but this is not relevant as far as I can tell; the issue is that a String value ends up being converted to a Number value).

Is there any way to avoid this String->Number conversion or some other way to make this test pass?