default_port() takes a scheme but not a protocol()

```cpp
auto const myurl = url("https://exampe.com");
url::default_port(myurl.protocol()).has_value(); // false
```

So, `protocol()` returns `https:` but `default_port()` needs a scheme (no ':'). Plus, there is no `url::scheme()` provided. Is this some overlooking ? Having to strip the ':' every time is annoying.