Url.to_file_path() doesn't produce UNC path on Windows

Url.to_file_path() doesn't produce a UNC path on Windows, even when the Url was initialized with a UNC path via Url.from_file_path(). Thus this program:

extern crate url;

use std::path::Path;
use url::Url;

fn main() {
	let unc_path = Path::new(r"\\?\C:\Windows\System");
	let url = Url::from_file_path(unc_path).expect("url");
	let abs_path_buf = url.to_file_path().expect("path");
	let abs_path = abs_path_buf.as_path();
	assert_eq!(unc_path, abs_path);
}

Fails with:

thread 'main' panicked at 'assertion failed: `(left == right)`
left: `"\\\\?\\C:\\Windows\\System"`,
right: `"C:\\Windows\\System"`', src\main.rs:11:2

It seems like Url.to_file_path() should produce a UNC path, at least when the Url was initialized with one; and perhaps in all cases, for compatibility with std::fs::canonicalize(), which always produces UNC paths on Windows (although this is controversial, per https://github.com/rust-lang/rust/issues/42869).

Start by running the supplied Windows example and reading the behavior of Url::from_file_path() and Url::to_file_path(). Trace how the UNC prefix is handled during the round trip. Done means a UNC path initialized with Url::from_file_path() is returned unchanged by to_file_path(), with the example assertion passing.