Regression in `set_scheme` for 2.1.1 vs 2.1.0

Cargo's internal test suite is unfortunately failing after the update to url 2.1.1, and one thing we've narrowed down so far is that the behavior of this changed between 2.1.0 and 2.1.1:

use url::Url;

fn main() {
    let mut url = Url::parse("git://github.com/foo/bar").unwrap();
    println!("{:?}", url.set_scheme("https"));
}

On 2.1.0 this succeeded but on 2.1.1 this is now failing.

Is this an intended change or perhaps an accidental bug? If it's intended, is there a way we can get this working?

Start with the supplied Rust reproduction and compare Url::set_scheme behavior between url 2.1.0 and 2.1.1. Read the set_scheme entry point and the Cargo test failure mentioned in the issue, then determine whether the changed result is intended; done means the compatibility question is resolved and the expected behavior is clear.