public function testURLsSetToNullIfInvalid()
{
$problem = new HTTPProblem(400, 'msg');
$problem->withType('http://example.com')->withInstance('http://example.com/page.html');
$this->assertSame('http://example.com', $problem->type());
$this->assertSame('http://example.com/page.html', $problem->instance());
$problem->withType('not-a-url')->withInstance('not-a-url-2');
$this->assertSame(null, $problem->type());
$this->assertSame(null, $problem->instance());
}
