strange behaviour with `url` crate

```rust
#[test]
fn x() {
    #[derive(Deserialize, Serialize)]
    #[serde(untagged)]
    pub enum L { L(Vec<url::Url>) }
    let h = L::L(vec!["file://".parse().unwrap()]);
    bendy::serde::from_bytes::<L>(&bendy::serde::to_bytes::<L>(&h).unwrap()).expect("rt ok");
}
```
this fails with 
```
CustomDecode("data did not match any variant of untagged enum L")
```
unsure why, seems like a bug.