protected function seeHeader($headerName, $value = null)
    {
        $headers = $this->response->headers;

        $this->assertTrue($headers->has($headerName), "Header [{$headerName}] not present on response.");

        if (! is_null($value)) {
            $this->assertEquals(
                $headers->get($headerName), $value,
                "Header [{$headerName}] was found, but value [{$headers->get($headerName)}] does not match [{$value}]."
            );
        }

        return $this;
    }