Undefined property when using multiple bulks

When I run the following code, I got a PHP notice :

```php
<?php
require_once __DIR__ . '/vendor/autoload.php';

use \ElasticSearch\Client;
$es = Client::connection('http://127.0.0.1:9200/myindex/mytype')

// Run multiple bulks
$client->beginBulk();
$client->index(array('title' => 'My bulked entry 1'));
$client->index(array('title' => 'My bulked entry 2'));
$client->commitBulk();

$client->beginBulk();
$client->index(array('title' => 'My bulked entry 3'));
$client->index(array('title' => 'My bulked entry 4'));
$client->commitBulk();
```



     Notice: Undefined property: ElasticSearch\Client::$bulk in /home/jc/github/elasticsearch/src/ElasticSearch/Client.php on line 302



This is because the Client::commitBulk() method unsets $this->bulk.
Then, in the second Client::beginBulk() call, the condition "if (!$this->bulk)" at line 302 fails because $this->bulk is undefined.

To fix this, I replaced the unset $this->bulk by setting $this->bulk to null.
I also add the corresponding test in Client::testBulk()