null without a comma at the end is Hocon::String("null"), not Hocon::null

[35VLG84]

Hello,
First of all, Thank You very much for implementing HOCON for rust!

There seem to be a following anomaly with null-values: If there isn't a comma after null value, it's interpreted as Hocon::String("null"), instead of Hocon::Null. By specification both cases should be null. This is also how Config does it (both are nulls), so this seems to be a bug.

use hocon::{Hocon, HoconLoader, Error};

fn check_null(doc: &Hocon) {
   match doc {
        Hocon::Null => {
            println!("It's null!");
        }
        _ => {
            println!("{:#?}", doc);
            panic!();
        }
   }
}

fn main() -> Result<(), Error> {
    // this works
    //let example = r#"{ null_with_comma = null, a { a_null = null, } }"#;
    let example   = r#"{ null_with_comma = null, a { a_null = null  } }"#;

    let doc = HoconLoader::new().load_str(example)?.hocon()?;

    check_null(&doc["null_with_comma"]);

    check_null(&doc["a"]["a_null"]);
    Ok(())
}

I'm planning to use hocon.rs for one project and noticed this while preparing an RFC pull request for hocon.rs. I will open the RFC pull request soon, so we can discuss about it separately.