hyper::Uri does not seem to parse URIs that do not have a host field

The Uri parser does not appear to be able to handle URIs that do not provide a host field.

I realize that the hyper::Uri module might not be intended to provide a complete URI implementation, so maybe just a mention of this in the docs is good enough.

Anyway, here's the issue: Omitting a host field causes the parser to fail with InvalidFormat.

For example, RFC 8089 defines the file schema, allowing local files to omit host. Both file:/// and file:/ are allowed. RFC 3986 does not require a host field after a scheme. But this test fails on the from_str().

    #[test]
    fn should_parse_file_scheme() {
        let uri = hyper::Uri::from_str("file:///foo/bar").expect("Should parse URI with no host");
        assert!(uri.host().is_none());
    }

Tested on Hyper 0.14.4

Locate hyper::Uri's from_str parser and its URI parsing tests, then reproduce the failure with file:///foo/bar and file:/ URIs. Determine whether the intended outcome is parser support or a documentation note; done means the chosen behavior is covered by a test or clearly documented.