Guzzle 4 in Drupal 8. Get a JSON version of a node.

rest-request.php

<?php

// Example using cookie auth.
print (string) \Drupal::httpClient()
  ->get('http://example.com/node/1', [
    'headers' => ['Accept' => 'application/json'], 
    'cookies' => ['SESS07e0668737e9f97189850d2cf0c79892' => 'dasGegKQ6ZzEF0xvRZWWzwrl9tt7UT8fGuXrAeLC9P1'],
  ])
  ->getBody(TRUE);

// Example using basic auth.
print (string) \Drupal::httpClient()
  ->get('http://example.com/node/1', [
    'headers' => ['Accept' => 'application/json'], 
    'auth' => ['username', 'password'], 
  ])
  ->getBody(TRUE);

// Creating a node.
$response = \Drupal::httpClient()
  ->post('http://example.com/entity/node', [
    'timeout' => 120,
    'auth' => ['admin', 'drupal'], 
    'body' => '{"type":[{"target_id":"article"}],"title":[{"value":"REST Create Node Test Title."}]}',
    'headers' => [
      'Accept' => 'application/json', 
      'Content-Type' => 'application/json'
    ], 
  ]);

print_r($response);

// Using HAL
$response = \Drupal::httpClient()
  ->post('http://example.com/entity/node', [
    // Username and password for HTTP Basic Authentication.
    'auth' => ['admin', 'drupal'],
    'body' => json_encode([
       'title' => [['value' => 'Example node title']],
       'type' => [['target_id' => 'article']],
       '_links' => ['type' => ['href' => 'http://example.com/rest/type/node/article']],
    ]),
    'headers' => [
      'Accept' => 'application/hal+json',
      'Content-Type' => 'application/hal+json'
    ],
  ])
  ->getStatusCode();