public function testCanSignExpiringUrl()
{
$valid_signed_url = $this->service->sign($this->url, '+1 day');
$invalid_signed_url = $this->service->sign($this->url, '-1 day');
$this->assertTrue($this->service->isValid($valid_signed_url));
$this->assertFalse($this->service->isValid($invalid_signed_url));
}