Generating URI for route fails with custom host formatter

[imbrish]

• Laravel Version: 5.7, 5.6, 5.5 (and earlier?)
• PHP Version: any
• Database Driver & Version: any

Description:

Generating relative URLs for routes doesn't work with custom host formatter.

Steps To Reproduce:

public function testGenerateRelativeUrlForRouteWithHostFormatter()
{
    $url = new UrlGenerator(
        $routes = new RouteCollection,
        $request = Request::create('http://www.foo.com/')
    );

    $routes->add(
        new Route(['GET'], '/named-route', ['as' => 'plain'])
    );

    $url->formatHostUsing(function ($host) {
        return str_replace('foo.com', 'foo.org', $host);
    });

    $this->assertEquals('http://www.foo.org/named-route', $url->route('plain'));
    $this->assertEquals('/named-route', $url->route('plain', [], false));
}

Second assertion fails:

Failed asserting that two strings are equal.
--- Expected
+++ Actual
@@ @@
-'/named-route'
+'/http://www.foo.org/named-route'

Reason

RouteUrlGenerator tries to remove original root from uri and fails, because root has been modified.