Change return type of password_mut to Option<&mut Option<String>>

[KiChjang]

This is necessary because by default, if the URL ':' delimiter was not found in the URL, then password_mut() would return None, causing a later mutation to the password field in RelativeSchemeData impossible.

[KiChjang]

r? @SimonSapin

[SimonSapin]

This is sensible, but I have an in-progress rewrite that makes it obsolete: https://github.com/servo/rust-url/compare/1.0 (The password and other components will not be independent Strings anymore. *_mut methods will be replaced with setters, though I’m not certain of the exact API yet.)

So making a breaking release if another one is coming soon may not be worth it. In the mean time, as discussed on IRC, you can use the Url::relative_scheme_data_mut() method and access the RelativeSchemeData::password field directly.

[KiChjang]

Ah ok, the rewrite makes more sense then.