Fix file URL serialization

[domenic]

The current bug we need to fix is that parsing /foo/bar against file:/path/to/docroot/index.html currently gives file:/foo/bar, whereas it should give file:///foo/bar.

In fact, just parsing file:/path/to/docroot/index.html should give back file:///path/to/docroot/index.html.

The complication here is that we have no tests for href serialization, which is horrible.

Related: whatwg/url#52 web-platform-tests/wpt#2038