host:port string with no scheme parses invalidly

Trying to parse a string such as a.local.com:3000 should fail, as it does not have a scheme. However, it is allowed to parse, and when .scheme() is called on the resulting Url struct. a.local.com is returned as the scheme. This can cause unexpected behavior and allow invalid URLs to slip through as valid.

This seems to be related to this issue.

Code Sample:

use url::Url;
fn main() {
    let url = Url::parse("a.local.com:3000").unwrap();
    println!("{}, scheme: {}", url.to_string(), url.scheme());
}

Output: a.local.com:3000, scheme: a.local.com

Start at Url::parse and reproduce the issue with the provided a.local.com:3000 example. Trace why the input is accepted with a.local.com as its scheme; done means scheme-less host:port strings are rejected while valid URLs continue to parse correctly.