URI parser cannot parse URNs

It seems the URI parser stumbles with URNs. A simple test to reproduce:

fn main() {
    use http::uri::Uri;
    let uri = "urn:isbn:1238492".parse::<Uri>();
    println!("{:?}", uri);
}

Produces the output:

$ ./testcase
Err(InvalidUri(InvalidAuthority))

The authority is only present when the scheme is followed by //, otherwise the parser should be parsing everything after the first : as the path. Python gets this right, and Go parses this as Opaque (instead of Host, but still successfully parses):

Python:

>>> urlparse('urn:isbn:0451450523')
ParseResult(scheme='urn', netloc='', path='isbn:0451450523', params='', query='', fragment='')

Go:

>>> url, _ := url.Parse("urn:isbn:0451450523")
>>> fmt.Println("Host (Opaque):", url.Opaque)
Host (Opaque): isbn:0451450523

Start with the Rust http::uri::Uri parsing entry point and reproduce the provided urn:isbn:1238492 example. Compare the parser's behavior with the stated URI rule and the Python and Go results. Done means a URN parses successfully and the text after the first colon is handled as path or opaque content rather than an invalid authority.