public function testGetHeaders()
 {
     $client = new Client('https://localhost:4010', ['Content-Type: application/json', 'Authorization: Bearer SG.XXXX']);
     $this->assertSame(['Content-Type: application/json', 'Authorization: Bearer SG.XXXX'], $client->getHeaders());
     $client2 = new Client('https://localhost:4010', null);
     $this->assertSame([], $client2->getHeaders());
 }