AK: Correctly set host when parsing URL with a base file:// host

We were completely missing this spec step here. Also leave a FIXME for
the pre-existing implementation of this step, as this doesn't match the
spec.

Tests/LibWeb/Text/expected/URL/url.txt

@@ -58,3 +58,13 @@ port => ''
 pathname => '/cat'
 search => '?dog'
 hash => '#meow%22woof'
+new URL('/hello', 'file://friends/')
+protocol => 'file:'
+username => ''
+password => ''
+host => 'friends'
+hostname => 'friends'
+port => ''
+pathname => '/hello'
+search => ''
+hash => ''

Tests/LibWeb/Text/input/URL/url.html

@@ -26,6 +26,7 @@
             { input: 'http://[1:1:0:0:1:0:0:0]/' },
             { input: 'unknown://serenityos.org:0' },
             { input: 'http://serenityos.org/cat?dog#meow"woof' },
+            { input: '/hello', base: 'file://friends/' },
         ]) {
             printURL(url.input, url.base);
         }