You can access the original ResponseInterface object via its getter.
public function testCtorDefaults()
{
$response = new Response();
$response = $response->withStatus(404, 'File not found');
$e = new ResponseException($response);
$this->assertEquals(404, $e->getCode());
$this->assertEquals('HTTP status code 404 (File not found)', $e->getMessage());
$this->assertSame($response, $e->getResponse());
}
